boards: arm: nrf5340pdk: remove un-needed nrf5340pdk prefix We do not need to have the nrf5340pdk_ prefix in the partition configuration and sram planning files for nrf5340 platforms because these files may be common for any nrf5340 board variant. Also, we do not need it in the common .dts header for the Application MCU. Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>