serial: stm32x7: simplify baud rate register calculation

Simplify baud rate register formula and use the oversampling
uart feature.
This code is aligned with what is implemented in kernel driver
drivers/tty/serial/stm32-usart.c since kernel v4.9.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Christophe KERELLO <christophe.kerello@st.com>
Reviewed-by: Patrick DELAUNAY <patrick.delaunay@st.com>
Acked-by: Vikas MANOCHA <vikas.manocha@st.com>