leds: gpio-leds: retain the LED status on suspend

Add support to retain the LED status on suspend.
Enable the feature by define retain-state-suspended
in the LED sub-node properties.

CRs-fixed: 649441
Change-Id: I254f84e5f859be8637d8b3220f32a1736a1f0c60
Signed-off-by: Guoping Yu <guopingy@codeaurora.org>