ARM: rockchip: add missing of_node_put in rockchip_smp_prepare_pmu

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-rockchip/platsmp.c:250:2-8: ERROR: missing of_node_put; acquired a node pointer with refcount incremented on line 241, but without a corresponding object release within this function.
./arch/arm/mach-rockchip/platsmp.c:260:2-8: ERROR: missing of_node_put; acquired a node pointer with refcount incremented on line 241, but without a corresponding object release within this function.
./arch/arm/mach-rockchip/platsmp.c:263:1-7: ERROR: missing of_node_put; acquired a node pointer with refcount incremented on line 241, but without a corresponding object release within this function.

Signed-off-by: Wen Yang <wen.yang99@zte.com.cn>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Heiko Stuebner <heiko@sntech.de>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-rockchip@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Heiko Stuebner <heiko@sntech.de>