pinctrl: elaborate a bit on arrangements in doc

This elaborates a bit on the pin control and pin muxing
logic vs GPIO arangements in the hardware.

Inspired by some drawings in a mail from Christian Ruppert.
Both arrangements are confirmed to exist in practice.

Cc: Rob Landley <rob@landley.net>
Reviewed-by: Christian Ruppert <christian.ruppert@abilis.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>