arm: vt8500: Convert irq.c for multiplatform integration

This patch converts arch-vt8500/irq.c to MULTI_IRQ_HANDLER and
SPARSE_IRQ. IRQ domain is changed from legacy to linear.

Also, remove legacy code in include/mach/entry-macro.S and
include/mach/irq.h to prepare for multiplatform.

Signed-off-by: Tony Prisk <linux@prisktech.co.nz>
Acked-by: Arnd Bergmann <arnd@arndb.de>