CHROMIUM: Input: atmel_mxt_ts - add sysfs entry to read info checksum

CHROMIUM: Input: atmel_mxt_ts - add sysfs entry to read info checksum

The device stores the Information Block Checksum in the 3 bytes
immediately following the Information Block.

Read these three bytes, cache the value, and add a sysfs entry so
userspace can read it.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
BUG=chrome-os-partner:9103
TEST=cat /sys/bus/i2c/drivers/atmel_mxt_ts/2-004b/info_csum

Change-Id: I27512c624bad15674bc7c50c15717913595affe4
Reviewed-on: https://gerrit.chromium.org/gerrit/20599
Reviewed-by: Benson Leung <bleung@chromium.org>
Commit-Ready: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Tested-by: Daniel Kurtz <djkurtz@chromium.org>