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>