remove gpio_irq as it cannot be supported by the platform

gpio_irq requires to be able to trigger on both rising and falling edges but
the CS-300 AHB GPIO is only able to trigger on either but not both. A workaround
could be to invert the sensibility in the handler but this would induce latency
and complexity in the driver that are not required for the time being.

Signed-off-by: Wilfried Chauveau <wilfried.chauveau@arm.com>