clk: stm32mp: handle ck_usbo_48m clock provided by USBPHYC

Handle the input clock of RCC USB_PHY_48, provided by USBPHYC
and named "ck_usbo_48m".

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