sysreset: Add support for gpio-restart

sysreset: Add support for gpio-restart

The Linux kernel has binding for gpio-restart node.
This patch is adding basic support without supporting any optional
properties.
This driver was tested on Microblaze system where gpio is connected to
SoC reset logic.
Output value is handled via gpios cells values.

In gpio_reboot_request() set_value is writing 1 because
dm_gpio_set_value() is capable to changing it when it is ACTIVE_LOW.
...
	if (desc->flags & GPIOD_ACTIVE_LOW)
		value = !value;
...

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>