ARMv8: fix error in freeing stack frame

relocate_code() allocates 32 bytes stack frame but only 16 bytes are
freed before return. it will cause errors to possible previous frames
and doesn't make relocate_code() look like a function.

fix by freeing 32 bytes stack space

Signed-off-by: zijun_hu <zijun_hu@htc.com>
Reviewed-by: Simon Glass <sjg@chromium.org>