bus: vexpress-config: add missing of_node_put after calling of_parse_phandle

of_node_put needs to be called when the device node which is got
from of_parse_phandle has finished using.

Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>