sunxi: mmc: Set CONFIG_SYS_MMC_MAX_DEVICE

Set CONFIG_SYS_MMC_MAX_DEVICE to 4 for sunxi SoC.
This define is needed in the API code.

Signed-off-by: Emmanuel Vadot <manu@bidouilliste.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>