gpio-button-hotplug: support interrupt properties

gpio-button-hotplug: support interrupt properties

Upstream Linux's input gpio-keys driver supports
specifying a external interrupt for a gpio via the
'interrupts' properties as well as having support
for software debounce.

This patch ports these features to OpenWrt's event
version. Only the "pure" interrupt-driven support is
left behind, since this goes a bit against the "gpio"
in the "gpio-keys" and I don't have a real device to
test this with.

This patch also silences the generated warnings showing
up since 4.14 due to the 'constification' of the
struct gpio_keys_button *buttons variable in the
upstream struct gpio_keys_platform_data declaration.

gpio-button-hotplug.c: In function 'gpio_keys_get_devtree_pdata':
gpio-button-hotplug.c:392:10: warning: assignment discards 'const'
	qualifier from pointer target type [-Wdiscarded-qualifiers]
   button = &pdata->buttons[i++];
          ^
gpio-button-hotplug.c: In function 'gpio_keys_button_probe':
gpio-button-hotplug.c:537:12: warning: assignment discards 'const'
	qualifier from pointer target type [-Wdiscarded-qualifiers]
   bdata->b = &pdata->buttons[i];
            ^
gpio-button-hotplug.c: In function 'gpio_keys_probe':
gpio-button-hotplug.c:563:37: warning: initialization discards 'const'
	qualifier from pointer target type [-Wdiscarded-qualifiers]
   struct gpio_keys_button *button = &pdata->buttons[i];
                                   ^
Acked-by: Petr Štetiar <ynezz@true.cz>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>