Add driver for AM33xx SoC touchscreen

Split ADC driver in two halves: ADC(analog ot digital) and
TSC(touchscreen). Touchscreen driver is fully functional
up to the point of reporting samples. This part will be added
once FreeBSD has API for touchscreen.

Tested on: Beaglebone Black + 4DCAPE-43T
Reviewed by: loos
Differential Revision: https://reviews.freebsd.org/D5847