drivers: intc: plic: define all registers' offset in the driver

Define all the register offset directly in the driver according
to the RISCV PLIC specification as they are not configurable,
see: https://github.com/riscv/riscv-plic-spec

.

Updated devicetrees that has PLIC accordingly.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>