MFC r349319, r349328-r349330

r349319:
Add some i2c slave-device drivers that were missing from NOTES.

r349328:
The gpiopps(4) driver currently has probe and attach code only for FDT based
systems, so conditionalize it accordingly in conf/files.

r349329:
Add gpio(4) and related drivers to NOTES.

r349330:
Add pwm devices to NOTES.