irqchip/meson: Add support for gpio interrupt controller

Add support for the interrupt gpio controller found on Amlogic's meson
SoC family.

This controller is a separate controller from the gpio controller. It is
able to spy on the SoC pad. It is essentially a 256 to 8 router with a
filtering block to select level or edge and polarity. The number of actual
mappable inputs depends on the SoC.

Cc: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>