dts: nordic: add QSPI XIP area registers for nordic devices

There is currently no information available about the XIP
addresses for the nRF52840 and nRF5340.

Add this via a new 'reg' block in the relevant QSPI nodes, along with
names for the register blocks so they are easier to retrieve.

Signed-off-by: Håkon Øye Amundsen <haakon.amundsen@nordicsemi.no>
Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>