dts: nordic: Change compatible property for EXMIF node

This is a follow-up to commit cdf45cb234077522b5cef2da084869af43d42dc1.

Adjust the DTS node for the nRF EXMIF peripheral so that it is possible
to handle the peripheral with the generic MSPI driver for DW SSI based
controllers and use all its data lines in communication.
Also adjust the related board files accordingly.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>