Some mundane tweaks and cleanups to help de-clutter the diffs of some
upcoming functional changes.

Add an ofw_compat_data table for probing compat strings, and use it to add
PNP data. Remove some stray semicolons at the end of macro definitions,
and add a PWM_LOCK_ASSERT macro to round out the usual suite. Move the
device_t and driver_methods structs to the end of the file. Tweak comments.