ARM: smp: fix incorrect per-cpu definition of regs_before_stop

The commit efd00289 introduced changes
to store CPU registers for all CPUs that handle IPI_CPU_STOP. The
structure to save the registers was intended to be a per-cpu variable.
However, the patch did not allocate a per-cpu structure and instead only
ended up providing a compiler per-cpu directive. Fix this bug by actually
defining a static per-cpu variable.

Change-Id: Ifd192223c135025897e10312f61b108757575068
Signed-off-by: Syed Rameez Mustafa <rameezmustafa@codeaurora.org>