ARM: SMP: ensure frame pointer is reinitialized for soft-CPU hotplug

When we soft-CPU hotplug a CPU, we reset the stack pointer and
jump back to start_secondary().  This allows us to restart as if
the CPU was actually reset.

However, we weren't resetting the frame pointer, which could cause
problems with backtracing.  Reset the frame pointer to zero (which
means no parent frame) just like the early assembly code also does.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>