[CORE] vmm_devtree: Add CPU node attributes for DTB-based SMP core bringup

Going forward we would like to have ful DTB-based secondary core bringup
for SMP host. This means our architecture specific code would get info
required for bringup secondary cores from CPU nodes under /cpus.

Signed-off-by: Anup Patel <anup@brainfault.org>