dts: arm: nordic: rename nrf5340_cpuapp_common.dtsi

This commit renames the nrf5340_cpuapp_common.dtsi header
to nrf5340_cpuapp_peripherals.dtsi. This name is more
appropriate, as the header contains DT definitions for
peripherals, accessible from either Secure or Non-secure
execution environment (i.e. not accessible exclusively
by Secure or Non-Secure).

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