sunxi: Add mmc card-detect functionality

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
sunxi: Turn MMC_SUNXI_SLOT_EXTRA into a proper Kconfig option

Note we also drop the SPL check for initializing the 2nd mmc slot, the SPL
check is not necessary with Kconfig, because only options explicitly marked
as also being for the SPL get set during SPL builds.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
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>
sunxi: Use PG3 - PG8 as io-pins for mmc1

None of the known sunxi devices actually use mmc1 routed through PH, where
as some devices do actually use mmc1 routed through PG, so change the routing
of mmc1 to PG. If in the future we encounter devices with mmc1 routed through
PH, we will need to change things to be a bit more flexible.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
sunxi: Enable second sdcard slot found on some boards

Enable the second sdcard slot found on some boards. Note that we do not
set CONFIG_MMC_SUNXI_SLOT_EXTRA for the SPL, as having it there is not useful,

Except for on the Mele-M3 where the second sdcard is an eMMC, from which the
device can also boot, and there we want to have both in the SPL, so that
a single u-boot binary can both from both. So for the M3 we do prefix the
defconfig setting with the special "+S:" syntax so that it applies to the
SPL too.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>