drivers: counter: NXP SNVS rtc: Add support for NXP imx SNVS RTC

Adds a driver using the SNVS high power and optionally low power
RTC instances. A device specific function `mcux_snvs_rtc_set` is
provided to update the current counter value.

Signed-off-by: Pieter De Gendt <pieter.degendt@basalte.be>