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>
Acked-by: Ian Campbell <ijc@hellion.org.uk>
diff --git a/configs/Mele_M3_defconfig b/configs/Mele_M3_defconfig
index 645b236..a043ad2 100644
--- a/configs/Mele_M3_defconfig
+++ b/configs/Mele_M3_defconfig
@@ -1,5 +1,7 @@
 CONFIG_SPL=y
 CONFIG_SYS_EXTRA_OPTIONS="MELE_M3,AXP209_POWER,SUNXI_GMAC,USB_EHCI"
 CONFIG_FDTFILE="sun7i-a20-m3.dtb"
++S:CONFIG_MMC_SUNXI_SLOT_EXTRA=2
++S:CONFIG_MMC0_CD_PIN="PH1"
 +S:CONFIG_ARM=y
 +S:CONFIG_TARGET_SUN7I=y