ARM: dts: sunxi: Enable UEXT related nodes for Olimex A20 SOM EVB

UEXT are Universal EXTension connector from Olimex. They embed i2c, spi
and uart pins along power in one connector and are found on most,
if not all, Olimex boards.
The Olimex A20 SOM EVB have two UEXT connector so enable the nodes found on
those two connectors.

Signed-off-by: Emmanuel Vadot <manu@bidouilliste.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>