pinctrl.txt: move it to the driver-api book

As pinctrl describes a feature from drivers/base, place it at
the right book.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/driver-api/index.rst

@@ -37,6 +37,7 @@ available subsections can be seen below.
    80211/index
    uio-howto
    firmware/index
+   pinctl
    misc_devices
 
 .. only::  subproject and html