arch: mips: generic: Add missing of_node_put() in board-ranchu.c

In ranchu_measure_hpt_freq(), of_find_compatible_node() will return
a node pointer with refcount incremented. We should use of_put_node()
when it is not used anymore.

Signed-off-by: Liang He <windhl@126.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>