powerpc/fsl-booke: Move loadcam_entry back to asm code to fix SMP ftrace

When we build with ftrace enabled its possible that loadcam_entry would
have used the stack pointer (even though the code doesn't need it).  We
call loadcam_entry in __secondary_start before the stack is setup.  To
ensure that loadcam_entry doesn't use the stack pointer the easiest
solution is to just have it in asm code.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>