remoteproc/keystone: Switch to using gpiod API

remoteproc/keystone: Switch to using gpiod API

This patch switches the driver away from legacy gpio/of_gpio API to
gpiod API, and removes use of of_get_named_gpio_flags() which I want to
make private to gpiolib.

Note that there is a behavior change in the driver: previously the
driver did not actually request GPIO, it simply parsed GPIO number out
of device tree and poked at it.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/Yxe20ehiOnitDGus@google.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>