MFC r350015-r350016

r350015:
Fix nxprtc(4) on systems that support i2c repeat-start correctly.

An obscure footnote in the datasheets for the PCx2127, PCx2129, and
PCF8523 rtc chips states that the chips do not support i2c repeat-start
operations. When the driver was originally written and tested, the i2c
bus on that system also didn't support repeat-start and just quietly
turned repeat-start operations into a stop-then-start, making it appear
that the nxprtc driver was working properly.

The repeat-start situation only comes up on reads, so instead of using
the standard iicdev_readfrom(), use a local nxprtc_readfrom(), which is
just a cut-and-pasted copy of iicdev_readfrom(), modified to send two
separate start-data-stop sequences instead of using repeat-start.

r350016:
In nxprtc(4), use the countdown timer for better timekeeping resolution
on PCx2129 chips too.

The datasheet for the PCx2129 chips says that there is only a watchdog
timer, no countdown timer. It turns out the countdown timer hardware is
there and works just the same as it does on a PCx2127 chip, except that you
can't use it to trigger an interrupt or toggle an output pin. We don't need
interrupts or output pins, we only need to read the timer register to get
sub-second resolution. So start treating the 2129 chips the same as 2127.