ARM: S3C64XX: Provide for board-specific IRQs

Set up some IRQ space to allocation to off-SoC interrupt controllers.
Default this to 16 IRQs. If individual boards require more than this
then they will need to modify this file so allocating a small number
helps reduce the number of modifications required.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>