soc: nordic: Add initial support for nRF54H20 EngA

Add nrfx and Kconfig related infrastructure plus SoC initialization
code to allow building for nRF54H20 targets (Application and Radio
cores).

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>
Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>