Adds the ADC driver for TI AM3xxx SoC family.

The ADC has a 12bit resolution and its raw output can be read via sysctl(8)
interface.

The driver allows the setup of ADC clock, samples average and open delay
(the number of clock cycles to wait before start the conversion).

The TSC_ADC module is set in the general purpose mode (no touchscreen
support).

Tested on Beaglebone-black.

Written based on AM335x TRM.

Reviewed by:	rpaulo
Approved by:	adrian (mentor)
Tested by:	me, Brian J. McGovern, Sulev-Madis Silber (ketas)