Input: atmel_mxt_ts - add sysfs entries to read fw and hw version

Make firmware and hardware version strings available to userspace.
This is useful, for example, to allow a userspace program to implement
a firwmare update policy.

Change-Id: I1eddb4bbf5f3f9ae6947a8528598973ddead18cf
Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Henrik Rydberg <rydberg@euromail.se>