sunxi: Use BROM stored boot_media value to determine our boot-source

Now that we know that the BROM stores a value indicating the boot-source
at the beginning of SRAM, use that instead of trying to recreate the
BROM's boot probing.

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