dts: Add initial support for nRF54H20 EngA SoC

Add definition of the nRF54H20 SoC revision EngA with its Application,
Radio, and Peripheral Processor (PPR) cores and basic peripherals:
GRTC, GPIOs, GPIOTE, and UARTs.

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