CHROMIUM: Input: atmel_mxt_ts - add calibrate sysfs entry

CHROMIUM: Input: atmel_mxt_ts - add calibrate sysfs entry

In some situations, a touch surface may get out of calibration.
It is useful to provide a means for userspace to trigger a recal, so add a
calibrate sysfs entry.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
BUG=chrome-os-partner:9148
TEST=echo 1 > /sys/bus/i2c/drivers/atmel_mxt_ts/<dev>/calibrate

Change-Id: Idbdd7bf4f412f03e36604a2a1fae162f059234d3
Reviewed-on: https://gerrit.chromium.org/gerrit/20812
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Tested-by: Daniel Kurtz <djkurtz@chromium.org>
Commit-Ready: Daniel Kurtz <djkurtz@chromium.org>