sunxi: Fix sun5i mbus speed when booting old kernels

Older linux-sunxi-3.4 kernels override our PLL6 setting with 300 MHz,
halving the mbus frequency, so set it to 300 MHz ourselves and base the
mbus divider on that.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>