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>