boards: arm: stm32: add pinctrl state name for UART peripheral

Add the pinctrl state name (default) for the UART/USART/LPUART
peripherals. Changes performed using the following Python script run
from the repository root:

```
from pathlib import Path
import re

for fpath in Path(".").glob("boards/arm/**/*.dts"):
    lines = open(fpath).readlines()

    is_stm32 = False
    for line in lines:
        if "stm32" in line:
            is_stm32 = True
            break

    if not is_stm32:
        continue

    with open(fpath, "w") as f:
        for line in lines:
            f.write(line)

            m = re.match(r"(\s+)pinctrl-0.*us?art.*", line)
            if m:
                f.write(m.group(1) + "pinctrl-names = \"default\";\n")
```

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>