gpio: mb86s7x: Enable ACPI support

Make the mb86s7x GPIO block discoverable via ACPI. In addition, add
support for ACPI GPIO interrupts routed via platform interrupts, by
wiring the two together via the to_irq() gpiochip callback.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>