boards: thingy53: Fix missing entry for CPUNET

CPUNET uses a diffetent DTS file and the edge connector entry was
missed, this causes build issue for CPUNET.

Fixes 017ff784("boards: thingy53: Update DTS files to support
expansion boards").

Signed-off-by: Chaitanya Tata <Chaitanya.Tata@nordicsemi.no>