pncconf: dont use hm2 pet_watchdog

The hm2 write function pets the watchdog now.
configs: remove pet_watchdog from smithy configs

pet_watchdog is no longer needed, the hm2 write function pets the
watchdog now.
configs: dont call hm2 read_gpio/write_gpio functions needlessly

The regular hm2 read/write functions read and write everything, including
gpios, and are already called in these hal files.

The hm2 read_gpio/write_gpio functions are intended to be faster,
reduced-capability functions for use in the fast thread.
docs: update hm2 documentation

This removes mention of the pet_watchdog function, and updates some
out-of-date information about the watchdog.