hw/char: Implement STM32L4x5 USART skeleton

hw/char: Implement STM32L4x5 USART skeleton

Add the basic infrastructure (register read/write, type...)
to implement the STM32L4x5 USART.

Also create different types for the USART, UART and LPUART
of the STM32L4x5 to deduplicate code and enable the
implementation of different behaviors depending on the type.

Signed-off-by: Arnaud Minier <arnaud.minier@telecom-paris.fr>
Signed-off-by: Inès Varhol <ines.varhol@telecom-paris.fr>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20240329174402.60382-2-arnaud.minier@telecom-paris.fr
[PMM: update to new reset hold method signature;
 fixed a few checkpatch nits]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>