dts: arm: nordic: move number of MPU regions in variant-specific file

Move the definition of the number of MPU regions
(num-mpu-regions) in a SoC variant-specific header
instead of the common .dtsi for nRF5340 Application
MCU. This allows us to define different SoC variants
with different number of MPU regions available.

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