[media] Input: atmel_mxt_ts - read touchscreen size

The touchscreen may have a margin where not all the matrix is used. Read
the parameters from T9 and T100 and take account of the difference.

Note: this does not read the XORIGIN/YORIGIN fields so it assumes that
the touchscreen starts at (0,0)

Signed-off-by: Nick Dyer <nick@shmanahar.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>