pinctrl: hide subsystem from the populace

Machines that have embedded pin controllers need to select them
explicitly, so why broadcast their config options to menuconfig.
We provide a helpful submenu for those machines that do select
it, making it possible to enable debugging for example.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>