ARM: shmobile: 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-shmobile/pm-rcar-gen2.c:77:2-8: ERROR: missing of_node_put; acquired a node pointer with refcount incremented on line 66, but without a corresponding object release within this function.
./arch/arm/mach-shmobile/pm-rcar-gen2.c:85:2-8: ERROR: missing of_node_put; acquired a node pointer with refcount incremented on line 66, but without a corresponding object release within this function.
./arch/arm/mach-shmobile/pm-rcar-gen2.c:90:2-8: ERROR: missing of_node_put; acquired a node pointer with refcount incremented on line 66, but without a corresponding object release within this function.

Signed-off-by: Wen Yang <wen.yang99@zte.com.cn>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>