gpio: of: Add documentation of new sleep standard GPIO specifiers

Add documentation of new GPIO specifiers indicating if the state of an
output pin should be maintained during sleep/low-power mode.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Acked-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>