dts: arm: nordic: add Eng A suffix to nRF5340 CPUAPP .dtsi headers

Rename the nRF5340 QKAA variant .dtsi headers (Application
MCU) by adding an Engineering A (Eng A) suffix. This is done
to indicate that the headers should be explicitly used when
building for a board that contains the Engineeering A nrf5340
SoC variant.

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