power: poweroff: gpio: convert to use descriptors

This switches the GPIO poweroff driver to use GPIO descriptors
rather than numeral GPIOs. We get rid of the specific inversion
handling as GPIO descriptors know if they are active low or
high and can assert the line properly, so we do not need to
check the flag OF_GPIO_ACTIVE_LOW returned from the old call
of_get_gpio_flags() anymore.

Also convert to use managed resources and use dev_* message
printing while we're at it.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>