powerpc/83xx: Add missing of_node_put() in mpc832x_spi_init()

In mpc832x_spi_init(), 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/20220621080932.4081935-1-windhl@126.com