phy: stm32-usbphyc: add counter of PLL consumer

Add the counter of the PLL user n_pll_cons managed by the 2 functions
stm32_usbphyc_pll_enable / stm32_usbphyc_pll_disable.

This counter allow to remove the function stm32_usbphyc_is_init
and it is a preliminary step for ck_usbo_48m introduction.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>