clock_imx8mq: Make frac_pll_init() static

Since frac_pll_init() is only used in this file, change it to 'static'.

This fixes the following sparse warning:

arch/arm/mach-imx/imx8m/clock_imx8mq.c:662:5: warning: no previous
prototype for ‘frac_pll_init’ [-Wmissing-prototypes]

Signed-off-by: Pedro Jardim <jardim.c.pedro@gmail.com>

arch/arm/mach-imx/imx8m/clock_imx8mq.c

@@ -636,7 +636,7 @@ void dram_pll_init(ulong pll_val)
 		;
 }
 
-int frac_pll_init(u32 pll, enum frac_pll_out_val val)
+static int frac_pll_init(u32 pll, enum frac_pll_out_val val)
 {
 	void __iomem *pll_cfg0, __iomem *pll_cfg1;
 	u32 val_cfg0, val_cfg1;