riscv: sim.cc: Consider cpu-map node in cpus node

cpu-map node may be present in cpus node.
https://github.com/torvalds/linux/commit/124e46a86580c71e0eee8459c5da7649318118db


So this commit adds logic to consider that by skipping cpu-map node.

Signed-off-by: Tan En De <ende.tan@starfivetech.com>