[ARM] Fix SMP irqflags support

The IRQ changes a while back broke the build for SMP machines.
Fix up the SMP code to use set_irq_regs/get_irq_regs as
appropriate.  Also, fix a warning in arch/arm/kernel/time.c
where 'regs' becomes unused for SMP builds.

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