sunxi: Add support for the Mele M3 board

The Mele M3 is yet another Allwinnner based Android top set box from Mele.

It uses a housing similar to the A2000, but without the USM sata storage slot
at the top.

It features an A20 SoC, 1G RAM, 4G eMMC (unique for Allwinner devices),
100Mbit ethernet, HDMI out, 3 USB A receptacles, VGA, and A/V OUT connections.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
sunxi: Add sunxi_gpio_get_val function

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

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
M3_wip
wip

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