irqchip: vt8500: Convert arch-vt8500 to new irqchip infrastructure

This patch moves the arch-vt8500 irq code to drivers/irqchip and converts
it to use the new IRQCHIP_DECLARE and irqchip_init. This allows the removal
of some more functions from common.h

Signed-off-by: Tony Prisk <linux@prisktech.co.nz>
Signed-off-by: Olof Johansson <olof@lixom.net>