ARM: 7123/1: smp: Add an IPI handler callable from C code

In order to be able to handle IPI directly from C code instead of
assembly code, introduce handle_IPI(), which is modeled after handle_IRQ().

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>