ARM: dts: sun7i: cubietruck: set mmc3 bus-width property

bus-width defaults to 1, and all 4 lines are hooked up at the cubietruck,
properly set bus-width to 4.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>