serial: stm32x7: add clk_get_rate() support

Replace proprietary clock_get() by clk_get_rate()
The stm32x7 serial driver is now "generic" and can be used
by other STM32 SoCs.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Acked-by: Vikas MANOCHA <vikas.manocha@st.com>