sunxi: add initial clock setup for sun9i for SPL

This is a cleaned up version set_pll() from Allwinner's boot0 source
(bootloader/basic_loader/bsp/bsp_for_a80/common/common.c).

[wens@csie.org: Added commit message; style cleanup]
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>