imx8m: clock_imx8mm: Staticize functions

Functions fracpll_configure(), decode_intpll(), decode_fracpll(),
get_root_src_clk() and get_root_clk() are used only in the scope of this
file, so make them static to fix the following sparse warnings:

arch/arm/mach-imx/imx8m/clock_imx8mm.c:50:5: warning: no previous
prototype for ‘fracpll_configure’ [-Wmissing-prototypes]
arch/arm/mach-imx/imx8m/clock_imx8mm.c:271:5: warning: no previous
prototype for ‘decode_intpll’ [-Wmissing-prototypes]
arch/arm/mach-imx/imx8m/clock_imx8mm.c:418:5: warning: no previous
prototype for ‘decode_fracpll’ [-Wmissing-prototypes]
arch/arm/mach-imx/imx8m/clock_imx8mm.c:483:5: warning: no previous
prototype for ‘get_root_src_clk’ [-Wmissing-prototypes]
arch/arm/mach-imx/imx8m/clock_imx8mm.c:527:5: warning: no previous
prototype for ‘get_root_clk’ [-Wmissing-prototypes]

Signed-off-by: Alifer Moraes <alifer.wsdm@gmail.com>