Input: atmel_mxt_ts - initialise input slots with INPUT_MT_DIRECT

This indicates the device coordinates should be directly mapped to screen.
This is valid since scaling/flipping/rotation should be done by configuring
the MXT device.

It also flags to Android using INPUT_PROP_DIRECT that the device should be
treated as a touch screen by default, and removes the necessity for a default
IDC file.

Signed-off-by: Nick Dyer <nick.dyer@itdev.co.uk>