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

ARM: mvebu: 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-mvebu/pm-board.c:135:2-8: ERROR: missing of_node_put; acquired a node pointer with refcount incremented on line 88, but without a corresponding object release within this functio

Signed-off-by: Wen Yang <wen.yang99@zte.com.cn>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Gregory Clement <gregory.clement@bootlin.com>
Cc: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>