drivers: serial: uart_stm32: convert to DT_INST defines

Convert driver to use DT_INST_ defines.
In the process, we've removed all UART/USART/LPUART code in
device instanciation code which had no impact.
Since all uart/usart/lpuart nodes declare compatibility with
st,stm32uart, DT_INST_X_ST_STM32_UART_FOO could be used.

Removed DT_UART fixup macros.

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