drivers/serial: stm32: Optimize LPUART code

On STM32 series currently available in Zephyr,
LPUART and U(S)ART IPs are similar and share most of their
registers. As a consequence LL API defined for U(S)ART also
applies to LPUART.
This allows to remove specific LPUART code.

Restrict use of LL LPUART API when it diverges from UART one.


Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>