rtc: rtc-rpi: Add simple RTC driver for Raspberry Pi

This supports setting and reading the real time clock
and supports wakeup alarms.

To support wake up alarms you want this bootloader config:
 POWER_OFF_ON_HALT=1
 WAKE_ON_GPIO=0

You can test with:
  echo +600 | sudo tee /sys/class/rtc/rtc0/wakealarm
  sudo halt

That will halt (in an almost no power state),
then wake and restart after 10 minutes.

Signed-off-by: Dom Cobley <popcornmix@gmail.com>