leds: leds-gpio: add retain-state-suspended property

Some gpio-leds need retain the state even in suspend, such as charger led.
But this property missed in devicetree, add it.

(cooloney@gmail.com: fold DT binding updates into this patch)

Signed-off-by: Robin Gong <b38343@freescale.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>