ARM: smp: Fix Unknown IPI message 0x1

Commit 'ad3b6993' converted ARM smp_cross_call() to take IPI number
as a parameter to handle more event than SGI and do_IPI was suppose
to recover SGI number. But the do_IPI doesn't consider it and it's
getting detected as 'Unknown IPI message 0x1' with ipi numbers are
moved to starts from 'IPI_TIMER=2"

There can be 16 different SGI but only SGI1 is used as IPI so
only that one is handled in do_IPI as IPI_CPU_START.

Added IPI_CPU_START because it wasn't used and thought it's
appropriate. Not sure whether its the right one.

Change-Id: I4dbe7c489d9611fbbb4036c15ac247659fde4119
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Cc: Russell King <rmk+kernel at arm.linux.org.uk>
[johlstei@codeaurora.org: fixed bounds check error]
Signed-off-by: Jeff Ohlstein <johlstei@codeaurora.org>