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>