sunxi: A20-Olimex-SOM-EVB defconfig: enable mmc3

The Olimex A20-SOM-EVB is an evaluation board for the Olimex
A20-SOM system-on-module. The baseboard provides a full-size SD
socket (connected to mmc3) in addition to the micro-SD socket on
the SOM itself (which is connected to mmc0).

Enable the mmc3 controller in the board defconfig.

Signed-off-by: Karsten Merker <merker@debian.org>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>