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: NPedro Jardim <jardim.c.pedro@gmail.com>

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

@@ -811,7 +811,7 @@ int clock_init(void)
  * Dump some clockes.
  */
 #ifndef CONFIG_SPL_BUILD
-int do_imx8m_showclocks(cmd_tbl_t *cmdtp, int flag, int argc,
+static int do_imx8m_showclocks(cmd_tbl_t *cmdtp, int flag, int argc,
 		       char * const argv[])
 {
 	u32 freq;