[CORE] devtree: Fix vmm_devtree_getchild() API

If we had two child nodes xyz and xyz_foo under abc node then
lookup for xyz_foo node under abc node may fail because current
vmm_devtree_getchild() does not try match all nodes. It may happen
that in this situation vmm_devtree_getchild() trys to match xyz
node first when looking for xyz_foo which will fail and it will
return NULL.

This patch fixes this issue.

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