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>