Documentation: rename pinctl to pin-control

pinctl is not ideal as pinctrl (with an 'r') is much more common. Linus
state that pin-control.rst would be the best name for the documentation.

Link: https://lore.kernel.org/linux-gpio/20210126050817.GA187797@x1/#t


Suggested-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Drew Fustini <drew@beagleboard.org>
Link: https://lore.kernel.org/r/20210302053059.1049035-4-drew@beagleboard.org


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