arch/arm/ameba: add shared RTC driver for RTL8721Dx / RTL8720F / RTL8721F

[dcgong2917]

Summary

Adds an on-chip RTC driver for the Realtek Ameba family, exposed as a
/dev/rtc0 date/time character device with alarm support. The driver lives in
the shared layer arch/arm/src/common/ameba/ameba_rtc.c and is wired into three
ICs — rtl8721dx , rtl8720f and rtl8721f — reusing the common code unchanged.

Design notes:

• Sits on the SDK fwlib RTC API (mirrored structures + local externs, no vendor
  headers pulled into the NuttX include world). The fwlib RTC API lives in the
  RAM source ameba_rtc.c, added to the fwlib build under CONFIG_AMEBA_RTC.
• Provides both the lower-half RTC (ameba_rtc_lowerhalf; alarm
  set/cancel/relative + upper-half callback) and the arch date/time hooks
  (up_rtc_initialize/getdatetime/settime, g_rtc_enabled), so NuttX
  system time is seeded from the RTC.
• The hardware keeps a year plus a day-of-year (no month/day register); the
  driver bridges that to the NuttX calendar with the libc UTC routines
  (timegm/gmtime_r), which is exact and reversible. Cold start seeds the
  build date when the stored year predates CONFIG_START_YEAR.
• The only per-chip fact — the RTC interrupt vector — lives in the per-chip
  ameba_rtc_chip.h (46 / 33 / 41); the shared driver is never edited for a new
  Ameba chip.

A rtc board profile (examples/alarm) is added to each board for verification,
and the board documentation is updated.

Impact

• New peripheral, opt-in via CONFIG_AMEBA_RTC (default n). No impact on
  existing configs.
• Affects rtl8721dx / rtl8720f / rtl8721f only.

Testing

• tools/checkpatch.sh -g master..HEAD: all checks pass.

• Builds and links cleanly on all three ICs (rtl8721dx, rtl8720f, rtl8721f).

• Hardware: verified on all three EVBs (rtl8721dx, rtl8720f, rtl8721f)
  with the rtc config, exercising every driver path:

  • Cold start / seeding: on first power-up the RTC reads back the seeded
    build date instead of a pre-epoch value, so date is valid immediately.
  • Read / set (RTC_RD_TIME / RTC_SET_TIME via NSH date): the
    year/day-of-year hardware model is correctly bridged to the full
    month/day calendar in both directions.
  • Timekeeping: consecutive date reads show the clock advancing in real
    time.
  • Alarm (RTC_SET_ALARM + IRQ callback via alarm example): a one-shot
    alarm fires once at the requested offset and does not re-fire.

  Representative NSH session (rtl8721dx):

    nsh> date
    Tue, Jun 16 00:00:07 2026            # seeded build date after cold start
    nsh> date -s "Jun 16 12:00:00 2026"
    nsh> date
    Tue, Jun 16 12:00:06 2026            # set time read back (+ elapsed seconds)
    nsh> date
    Tue, Jun 16 12:00:09 2026            # clock advancing
    nsh> alarm 5
    alarm_daemon: Running
    Opening /dev/rtc0
    Alarm 0 set in 5 seconds
    alarm_daemon: alarm 0 received       # fires once, 5 s later

[github-actions]

MemBrowse Memory Report

No memory changes detected for:

• arduino-mega2560
• esp32-devkitc
• hifive1-revb
• mirtoo
• qemu-armv8a
• qemu-intel64
• rx65n-rsk2mb
• s698pm-dkit
• stm32-nucleo-f103rb