docs/pinctrl: fix runtime pinmuxing example

The example declares "struct pinctrl *p" but refers to
"foo->p" which isn't declared in the context of the example.

Signed-off-by: Siarhei Volkau <lis8215@gmail.com>
Link: https://lore.kernel.org/r/20221101205159.1468069-3-lis8215@gmail.com


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