ARM: sunxi: fix a leaked reference by adding missing of_node_put

ARM: sunxi: fix a leaked reference by adding missing of_node_put

The call to of_get_next_child returns a node pointer with refcount
incremented thus it must be explicitly decremented after the last
usage.

Detected by coccinelle with the following warnings:
./arch/arm/mach-sunxi/mc_smp.c:110:1-7: ERROR: missing of_node_put; acquired a node pointer with refcount incremented on line 97, but without a corresponding object release within this functio
./arch/arm/mach-sunxi/platsmp.c:138:2-8: ERROR: missing of_node_put; acquired a node pointer with refcount incremented on line 129, but without a corresponding object release within this function

Signed-off-by: Wen Yang <wen.yang99@zte.com.cn>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Chen-Yu Tsai <wens@csie.org>
Cc: Russell King <linux@armlinux.org.uk>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org (open list)
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>