arm: orion: Use generic irq chip

The core interrupt chip is a straight forward conversion. The gpio
chip is implemented with two instances of the irq_chip_type which can
be switched with the irq_set_type function. That allows us to use the
generic callbacks and avoids the conditionals in them.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Nicolas Pitre <nico@fluxnic.net>