Fix sparse warning:

drivers/clk/zynqmp/divider.c:259:5: warning:
 symbol 'zynqmp_clk_get_max_divisor' was not declared. Should it be static?
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Link: https://lkml.kernel.org/r/20200403083040.37748-1-yuehaibing@huawei.comReviewed-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>