arm: bugfix: save_boot_params_default accesses uninitalized stack when -O0

save_boot_params_default() in cpu.c accesses uninitialized stack area
when it compiled with -O0 (not optimized).

Signed-off-by: Tetsuyuki Kobayashi <koba@kmckk.co.jp>
Acked-by: Tom Rini <trini@ti.com>