pinctrl: stm32: add possibility to configure pins individually

Adds the possibility to configure a single pin through the gpiolib (i.e:
to set PULL_UP/PULL_DOWN config).
Mutex behavior is slightly changed to avoid a deadlock when pin_config_set
is called (in this case pctldev->mutex is already taken).

Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>
Link: https://lore.kernel.org/r/20200615125951.28008-3-alexandre.torgue@st.com


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