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)
(1) Invoke cpp to bring in files via #include (although the old
/include/ stuff is supported still).
(2) bring in files from either vendor tree or freebsd-custom files
when building.
(3) move all dts* files from sys/boot/fdt/dts to
sys/boot/fdt/dts/${MACHINE} as appropriate.
(4) encode all the magic to do the build in sys/tools/fdt/make_dtb.sh
so that the different places in the tree use the exact same logic.
(5) switch back to gpl dtc by default. the bsdl one in the tree has
significant issues not easily addressed by those unfamiliar with
the code.
