ARM: riscpc: reduce IRQ handling code

Reduce the amount of IRQ handling code that RiscPC requires; there's no
need for this duplication if we place the virtual iomem base address for
each bank directly in the irq_data structure.  Provide helpers to get
the base address, and setup the base address and register mask.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>