clock_imx8mq: Remove the function sscg_pll_init()

Function sscg_pll_init() is not used anywhere, so it can simply be deleted.
This was found because of the following sparse
warning:

arch/arm/mach-imx/imx8m/clock_imx8mq.c:702:5: warning: no previous prototype for ‘sscg_pll_init’ [-Wmissing-prototypes]
 int sscg_pll_init(u32 pll)
     ^~~~~~~~~~~~~

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