Allow UART to be a source for wakeup from low power mode.

Tested on a MX27PDK by doing:

echo enabled > /sys/devices/platform/imx21-uart.0/tty/ttymxc0/power/wakeup

echo mem > /sys/power/state

and then pressing a key in the console will wakeup the sytem.
Suggested-by: NShawn Guo <shawn.guo@freescale.com>
Signed-off-by: NFabio Estevam <fabio.estevam@freescale.com>
Tested-by: NRichard Zhao <richard.zhao@freescale.com>
Signed-off-by: NShawn Guo <shawn.guo@linaro.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>