ARM: dts: sun7i: Olimex A20-SOM-EVB: Enable mmc3 (baseboard SD socket)

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 dts.

Signed-off-by: Karsten Merker <merker@debian.org>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>