[ARM] pxa/hx4700: fix hx4700 touchscreen pressure values

hx4700 touchscreen events were being dropped in ads7846_rx() because their
pressure values consistently exceeded the platform maximum of 512; a sample
of 256 pressure values were in the range 531 to 815.  Doubling the platform
maximum to 1024 allows hx4700 touchscreen events to pass the test.

Signed-off-by: Paul Parsons <lost.distance@yahoo.com>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>