Allow PXA IRQs to be numbered starting at 16, leaving 0 to 15 for the
ISA IRQs, if needed.

This patch depends on RMK's PXA_HAVE_BOARD_IRQS patch.

Signed-off-by: Marc Zyngier <marc.zyngier@altran.com>
Acked-by: Russel King <linux@arm.linux.org.uk>
Acked-by: Eric Miao <eric.y.miao@gmail.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>