ARM: sa1111: implement support for sparse IRQs

Implement the necessary allocation/freeing functionality to support
sparse IRQs with the SA-1111 device.  On non-sparse IRQ platforms,
this allows us to dynamically allocate from within the available IRQ
number space.

Acked-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>