powerpc/512x: Add missing of_node_put() in mpc5121_clk_init()

In mpc5121_clk_init(), of_find_compatible_node() will return a
node pointer with refcount incremented. The reference should be dropped
with of_node_put() when it is not used anymore.

Signed-off-by: Liang He <windhl@126.com>
[mpe: of_clk_add_provider() will take its own reference.]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220615143703.3968898-1-windhl@126.com