serial: stm32x7: prepare the ground to STM32F4 support

STM32F4 serial IP is similar to F7 and H7, but registers
are not located at the same offset and some feature are
only supported by F7 and H7 version.

Registers offset must be added for each version and also
some flags indicated the supported feature.

Update registers name to match with datasheet (sr to isr,
rx_dr to rdr and tx_dr to tdr) and remove unused regs
(cr2, gtpr, rtor, and rqr).

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>