Configure the analog input 7 which, on BBB, is connected to the 3V3B rail
through a voltage divisor (R163 and R164 on page 4 of BBB schematic).

Add a note about this on ti_adc(4) man page.  The ti_adc(4) man page will
first appear on 10.1-RELEASE.

MFC after:	1 week
Suggested by:	Sulev-Madis Silber (ketas)
Manual page reviewed by:	brueffer (D127)
Never, ever, abbreviate the month names on manual pages.

Pointed out by:	brueffer
Pointy hat to:	loos