Add RTC support to aplsmc(4). The SMC firmware distributed with macOS 12.x

Add RTC support to aplsmc(4).  The SMC firmware distributed with macOS 12.x
has a method to read the counter that forms the base of the RTC.  This seems
to be the preferred way to access the RTC going forward.  The RTC offset is
still stored in the SPMI PMU, but we can use the nvmem interface to read
and write that.  This makes the RTC work on systems with the M1 Pro/Max SoC.

Sprinkle some #ifdef SMALL_KERNEL around and enable the driver on RAMDISK
kernels.

ok patrick@