ARM: 6197/2: preliminary support for sparse IRQ

So to allow NR_IRQS to be dynamic and platforms to specify the number
of IRQs really needed.

Acked-by: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>