Enable UEXT related nodes for Olimex A20 SOM

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.

Patch has been applied upstream, in the meantime add the nodes to our custom
DTS.