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>

Documentation/devicetree/bindings/gpio/gpio.txt

@@ -80,6 +80,8 @@ Optional standard bitfield specifiers for the last cell:
            https://en.wikipedia.org/wiki/Single-ended_triode
 - Bit 2: 0 means open-source, 1 means open drain, see:
            https://en.wikipedia.org/wiki/Open_collector
+- Bit 3: 0 means the output should be maintained during sleep/low-power mode
+         1 means the output state can be lost during sleep/low-power mode
 
 1.1) GPIO specifier best practices
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