firmware: arm_scpi: 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 it.

Besides, of_address_to_resource always returns -EINVAL for error, delete
the assignment for ret.

Signed-off-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>