sm501: add power control callback

Add callback to get or set the power control if the device has the sleep
connected to some form of GPIO.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Cc: Arnaud Patard <apatard@mandriva.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>