dts: nordic: nrf5340_cpunet: fix number of MPU regions on Network MCU

In the SoC that comes on the nRF5340 DK the number of MPU
regions in the nRF5340 Network MCU is 8, instead of 16. We
fix this in DTS by introducing different .dtsi headers for the
different SoC part variants: the nrf340 Engineering A (first
SoC revision, present in the nRF5340 PDK) and the nrf5340
(second SoC revision, present on nRF5340 DK).

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>