ARM64: smp: fix incorrect per-cpu definition of regs_before_stop

The commit f3b4a40b 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: Iea7e52e91819f6f2c7f8d2c638545c0a68d2ef76
Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>