Document that #pinctrl-cells can be 1 or 2 for pinctrl-single,pins

Fixes: 27c90e5e ("ARM: dts: am33xx-l4: change #pinctrl-cells from 1 to 2")
Reported-by: Emmanuel Vadot <manu@bidouilliste.com>
Link: https://lore.kernel.org/linux-gpio/20210115190201.9273b637a7f967e7e55bc740@bidouilliste.com/
Cc: Tony Lindgren <tony@atomide.com>
Signed-off-by: Drew Fustini <drew@beagleboard.org>
Reviewed-by: Tony Lindgren <tony@atomide.com>
Link: https://lore.kernel.org/r/20210120050342.320704-1-drew@beagleboard.orgSigned-off-by: Linus Walleij <linus.walleij@linaro.org>