MFC r306430

Add touchscreen support for the official 7" RPi touch display

Technically touchscreen chip is FT5406 but all hardware
communication is performed by VideCore and only final results
are presented to ARM part through memory region shared between
VC and ARM.

evdev is used as userland interface. FT5406 supports up to
10 touchpoints, but for now driver emulates single touch device
because I do not have GUI bits to test this functionality.

Driver is not enabled in default config for RPI and RPI2

Tested with: evdev-dump, tslib