sunxi: Add missing boot_media fields in the SPL header

Commit b19236fd ("sunxi: Increase SPL header size to 64 bytes to avoid
code corruption") Added defines for MMC0 and SPI as boot identification.
After verifying on an OLinuXino Lime2 with NAND and eMMC, the expected
values have been confirmed and added to spl.h

Signed-off-by: Olliver Schinagl <oliver@schinagl.nl>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>