pinctrl: stm32: Add links to consumers

Using STM32 as guinea pig after Alex's initial positive
test to see if this is something we should encourage
in general and make default behaviour.

Cc: Benjamin Gaignard <benjamin.gaignard@st.com>
Cc:  Alexandre Torgue <alexandre.torgue@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>