ARM: mx51: Add support for low power suspend on MX51

Adds initial low power suspend functionality to MX51.
Supports "mem" and "standby" modes.

Tested on mx51-babbage.

Signed-off-by: Dinh Nguyen <Dinh.Nguyen@freescale.com>
Tested-by: Arnaud Patard <arnaud.patard@rtp-net.org>
Tested-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>