dts: Add initial support for nRF9280 SiP

Add definition of the nRF9280 SiP with its Application,
Radio, and Peripheral Processor (PPR) cores and a basic set
of peripherals: GRTC, GPIOs, GPIOTE, and UARTs and few others.

Signed-off-by: Emanuele Di Santo <emdi@nordicsemi.no>
Co-authored-by: Andreas Moltumyr <andreas.moltumyr@nordicsemi.no>