dt-bindings: rtc: Add Apple SMC RTC

dt-bindings: rtc: Add Apple SMC RTC

Apple Silicon Macs (M1, etc.) have an RTC that is part of the PMU IC,
but most of the PMU functionality is abstracted out by the SMC.
An additional RTC offset stored inside NVMEM is required to compute
the current date/time.

Reviewed-by: Mark Kettenis <kettenis@openbsd.org>
Reviewed-by: Neal Gompa <neal@gompa.dev>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Sven Peter <sven@kernel.org>
Signed-off-by: James Calligeros <jcalligeros99@gmail.com>
Reviewd-by: Mark Kettenis <kettenis@openbsd.org>
Link: https://patch.msgid.link/20251112-macsmc-subdevs-v5-1-728e4b91fe81@gmail.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>