sunxi: When we've both mmc0 and mmc2, detect from which one we're booting

sunxi SOCs can boot from both mmc0 and mmc2, detect from which one we're
booting, and make that one "mmc dev 0" so that a single u-boot binary can
be used for both the onboard eMMC and for external sdcards.

When we're booting from mmc2, we make it dev 0 because that is where the SPL
will load the tertiary payload (the actual u-boot binary in our case) from,
see: common/spl/spl_mmc.c, which has dev 0 hardcoded everywhere.

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