powerpc/512x: Add missing of_node_put() in clock-commonclk.c

In mpc5121_clk_provide_migration_support(), hold the reference returned
by of_find_compatible_node() and use it to call of_node_put() for refcount
balance.

Signed-off-by: Liang He <windhl@126.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220621080349.4081689-1-windhl@126.com