Input: imx_keypad - remove obsolete comment

Since commit 81e8f2bc (Input: imx_keypad - add pm suspend and resume
support) the imx_keypad driver supports power management, so let's remove
the obsolete comment.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>