csky: Support vmlinux bootup with MMU off

Modify SETUP_MMU macro to fit on both MMU-on or MMU-off enviornment
and vmlinux could bootup from MMU off enviornment for some cases.

Unify the style of _start and _start_smp_secondary in head.S to make
head.S looks more concise and easy to understand.

Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Cc: Arnd Bergmann <arnd@arndb.de>