plat-orion: share IRQ handling code

Split off Orion IRQ handling code into plat-orion/, and add
support for multiple sets of (32) interrupts.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Reviewed-by: Tzachi Perelstein <tzachi@marvell.com>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Nicolas Pitre <nico@marvell.com>