pinctrl: stm32: improve debugfs information of pinconf-pins entry

Print the name of the selected alternate function in addition to its
number. Ex:
   "pin 135 (PI7): alternate 10 (SAI2_FS_A) - ..."

Signed-off-by: Fabien Dessenne <fabien.dessenne@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>
Link: https://lore.kernel.org/r/20220502152524.283374-1-fabien.dessenne@foss.st.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>