serial: stm32x7: migrate serial struct to driver

This allow to remove include/dm/platform_data/serial_stm32x7.h
which was included in the past by stm32x7 driver and by
stm32f746-disco.c board file.
Since patch 42bf5e7c "serial: stm32f7: add device tree support"
this file is no more needed in board file.

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