MFC r305708, r305719

r305708:
Add evdev support to TI ADC/touchscreen driver

Add generic evdev support to touchscreen part of ti_adc: two absolute
coordinates + button touch to indicate pen position. Pressure value
reporting is not implemented yet.

Tested on: Beaglebone Black + 4DCAPE-43T + tslib

r305719:
Cleanup evdev support for TI ADC/TS

• evdev_set_methods call is not required if actual methods are no-ops
• evdev_set_serial is also optional if there is no meaningful input device identifier
• evdev_set_id on the other hand is mandatory, so set virtual bus with dummy vendor/product/version

Suggested by: Vladimir Kondratiev