CHROMIUM: Input: atmel_mxt_ts - add backupnv sysfs entry

CHROMIUM: Input: atmel_mxt_ts - add backupnv sysfs entry

Writing to the object sysfs entry permits individual object table entries
to be modified in the device RAM at runtime.  To permanently save
the settings, they must be written to Non-Volatile memory (NVM).
This patch adds a write-only sysfs entry to allow userspace to save
current settings to NVM, but restricts access to root.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
BUG=chromium-os:27713
TEST=ls -al /sys/.../backupnv
 -> --w------- 1 root root 4096 Feb 14 20:54 backupnv

Change-Id: Id767e0b7ef7f882d6a45ebfe7e96e02800ef2cb8
Reviewed-on: https://gerrit.chromium.org/gerrit/17945
Commit-Ready: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Tested-by: Daniel Kurtz <djkurtz@chromium.org>