CHROMIUM: Input: atmel_mxt_tx - add matrix_size sysfs entry

CHROMIUM: Input: atmel_mxt_tx - add matrix_size sysfs entry

Returns the number of X and Y sense lines.

This entry will be used by userspace for determining the dimensions of
the T37 arrays that will be returned by deltas and refs debugfs entries.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
BUG=chromium-os:29899
TEST=cat /sys/bus/i2c/drivers/atmel_mxt_ts/<dev>/matrix_size

Change-Id: I3f6414a1eaa09f51d345256e7f1bb90bdbbc528f
Reviewed-on: https://gerrit.chromium.org/gerrit/21458
Commit-Ready: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Tested-by: Daniel Kurtz <djkurtz@chromium.org>