drivers: serial: stm32: Add persistent uart_config struct

Add static uart_config struct to init macro with corresponding pointer
as uart_stm32_config struct member. This struct will store boot and
runtime UART configuration for reinitialization upon exiting low-power
modes, where register contents might be lost.

Remove hw_flow_control, parity, and baud_rate from uart_stm32_config and
uart_stm32_data structs, respectively, as the need for these is now
obviated by the presence of the uart_config struct, which contains
equivalent members.

Add default baudrate, parity, stop, and data bits constants, used to
initialize the uart_config struct, in case the corresponding UART DT
properties are not set.

Signed-off-by: Kenneth J. Miller <ken@miller.ec>