clk: imx: scu: Make pd_np with static keyword

Fix the following sparse warning:

./clk-scu.c:23:20: warning: symbol 'pd_np' was not declared. Should it be static?
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zou Wei <zou_wei@huawei.com>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

drivers/clk/imx/clk-scu.c

@@ -20,7 +20,7 @@
 #define IMX_SIP_SET_CPUFREQ		0x00
 
 static struct imx_sc_ipc *ccm_ipc_handle;
-struct device_node *pd_np;
+static struct device_node *pd_np;
 static struct platform_driver imx_clk_scu_driver;
 
 struct imx_scu_clk_node {