CHROMIUM: Input: atmel_mxt_ts - add debugfs infrastructure

CHROMIUM: Input: atmel_mxt_ts - add debugfs infrastructure

This patch just creates a per-device debugfs root directory.
Actual debugfs entries will be added in subsequent patches.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
BUG=chromium-os:29899
TEST=ls /sys/kernel/debug/atmel_mxt_ts
   => Should show directories for each atmel_mxt_ts device on the system

Change-Id: I82e5470f5d1658dae03ad8d66bf348cc4fb2edb2
Reviewed-on: https://gerrit.chromium.org/gerrit/21051
Commit-Ready: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Tested-by: Daniel Kurtz <djkurtz@chromium.org>