gpio: tps65910: Add sleep control support

The device tps65910/tps65911 supports the sleep
functionality in some of gpios. If gpio is configured
in output mode and sleep is enabled then during device
sleep state, the output of gpio becomes LOW regardless
of non-sleep output value.
Such gpio can be used to control regulator switch such
that output of regulator is off in device sleep state.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Linus Walleij <linus.walleij@linaro.org>