[CORE] vmm_devtree: Minor improvement to vmm_devtree_addnode() API

The vmm_devtree_addnode() API takes two argument: parent node and
new node name. The parent node argument can be NULL when calling
vmm_devtree_addnode() API.

This patch improves vmm_devtree_addnode() for the case when parent
node is NULL. When there is no root node and parent node is NULL,
the vmm_devtree_addnode() API will return new node with parent
as NULL. When root node is available and parent node is NULL, the
vmm_devtree_addnode() API will return new node with parent node
as root node.

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