boards: arm: nrf5340_dk: include soc.h in nrf5340_cpunet_reset.c

We need to include soc.h in nrf5340_cpunet_reset.c, to include
the nRF5340 device headers. Fixes a compilation error when we
build with CONFIG_ARM_MPU=n.

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