pinctrl: single: fix function name in documentation

pinctrl: single: fix function name in documentation

Use the correct the function name in the documentation for
"pcs_parse_one_pinctrl_entry()".

"smux_parse_one_pinctrl_entry()" appears to be an artifact from the
development of a prior patch series ("simple pinmux driver") which
transformed into pinctrl-single.

Fixes: 8b8b091bf07f ("pinctrl: Add one-register-per-pin type device tree based pinctrl driver")
Signed-off-by: Drew Fustini <drew@beagleboard.org>
Link: https://lore.kernel.org/r/20200617180543.GA4186054@x1
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>