arm: mach-k3: am62: Fixup CPU core, gpu and pru nodes in fdt

AM62x SoC is available in multiple variant:
- CPU cores (Cortex-A) AM62x1 (1 core), AM62x2 (2 cores), AM62x4 (4 cores)
- GPU AM625x with GPU, AM623x without GPU
- PRU (Programmable RT unit) can be present or not on AM62x2/AM62x4

Remove the relevant FDT nodes by reading the actual configuration
from the SoC registers, with that change is possible to have a single
dts/dtb file handling the different variant at runtime.
While removing GPU node and CPU nodes also the watchdog node
in the same Module Domain is removed.

A similar approach is implemented for example on i.MX8 and STM32MP1 SoC.

Signed-off-by: Emanuele Ghidoli <emanuele.ghidoli@toradex.com>
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>