pinctrl: stm32: Rework stm32_pconf_parse_conf()

pinctrl: stm32: Rework stm32_pconf_parse_conf()

Reduce the number of parameters of the function by moving inside
the function the decoding of the field 'config'.

While there:
- change the type of 'param' to 'unsigned int' to handle the extra
  values not in 'enum pin_config_param';
- change the type of 'arg' to 'u32' to avoid additional conversions
  and align to 'u32' the corresponding param of __stm32_gpio_set().

Signed-off-by: Antonio Borneo <antonio.borneo@foss.st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>