Input: omap4-keypad - switch to use managed resources

Now that input core supports devres-managed input devices we can clean
up this driver a bit.

Link: https://lore.kernel.org/r/X/qfJKiM21uyksYl@google.com


Tested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>