STM32U5 UART loses data with baud rates higher than 67800

[JohnK1987]

STM32U5 UART loses data with baud rates higher than 67800 - need to be verified.
This potential issue, based on #227, was created for easy track of the issue

We need:

• an example for reproducing
• someone with STM32U5 target for verification

[wdx04]

To use higher baud rates for serial communication it is required to disable deep sleep with the sleep_manager_lock_deep_sleep() function. Also, when DMA is used, it is possible to achieve very high baud rates without data loss. For example, with my BurstSerial library, I archived 10M baud rate (the theoretical maximum for 16x oversampling) on a NUCLEO-U545RE board, when communicating with the PC through the ST-LINK virtual COM port.

[lefebvresam]

Once I have some time, I will try to reproduce the issue, strip down my project en put an example online.