lib: utils/fdt: introduce fdt_node_is_enabled()

If an FDT node contains a "status" property and this property is not
"ok" or "okay", this node should be ignored. Introduce a function that
checks this.

Signed-off-by: Jan Remes <jan.remes@codasip.com>
Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Anup Patel <anup@brainfault.org>