input: atmel_mxt_ts: Add support for T58 object

T58 object can be used on some targets for extra noise
suppression.  It must be set as readable and writable
for configuration update to work properly.

Change-Id: I3743cb18d755e871b6fd659f607389a51466364d
Acked-by: Chun Zhang <chunz@qti.qualcomm.com>
Signed-off-by: Amy Maloche <amaloche@codeaurora.org>