Input: atmel_mxt_ts - add documentation for device tree

Signed-off-by: Nick Dyer <nick.dyer@itdev.co.uk>
Merge branch 'dev-maxtouch-v3.0' into maxtouch-v3.0
Input: Touchscreen:

atmel_mxt_ts supported INPUT_PROP_DIRECT

The INPUT_PROP_DIRECT property indicates that device coordinates should be
directly mapped to screen coordinates (not taking into account trivial
transformations, such as scaling, flipping and rotating). Non-direct input
devices require non-trivial transformation, such as absolute to relative
transformation for touchpads. Typical direct input devices: touchscreens,
drawing tablets; non-direct devices: touchpads, mice.