sunxi: Restore lowlevel_init usage

2 recent sunxi changes have removed the usage of lowlevel_init by moving some
code around and then setting CONFIG_SKIP_LOWLEVEL_INIT.
This is problematic for 2 reasons:

1) It does not just stop s_init from being called, it also stops
cpu_init_cp15 from getting called, which is undesirable.

2) We want u-boot.bin to be usable standalone, without SPL, some people e.g.
use an upstream u-boot.bin together with Allwinner's boot0 loader. So
u-boot.bin must (re)initialize the gpios, timer, etc.

This commit restores the lowlevel_init / s_init usage, while keeping the
changes to no longer use the global-data (gd) struct in the SPL.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>