clock_imx8mq: Make do_imx8m_showclocks() static

Since do_imx8m_showclocks() is only used inside this file, make it 'static'.

This fixes the following sparse warning:

arch/arm/mach-imx/imx8m/clock_imx8mq.c:836:5: warning: no previous prototype
 for ‘do_imx8m_showclocks’ [-Wmissing-prototypes]
 int do_imx8m_showclocks(cmd_tbl_t *cmdtp, int flag, int argc,
     ^~~~~~~~~~~~~~~~~~~

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