ARM: SMP: avoid using bitmasks and locks for IPIs, use hardware instead

Avoid using bitmasks and locks in the percpu area for IPIs, and instead
use individual software generated interrupts to identify the reason for
the IPI.  This avoids the problems of having spinlocks in the percpu
area.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>