pinctrl: Reformat documentation in dm/pinctrl.h

This normalizes the documentation to conform to kernel-doc style [1]. It
also moves the documentation for pinctrl_ops inline, and adds argument and
return-value documentation. I have kept the usual function style for these
comments. I could not find any existing examples of function documentation
inside structs.

[1] https://www.kernel.org/doc/html/latest/doc-guide/kernel-doc.html

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

doc/api/index.rst

@@ -9,6 +9,7 @@ U-Boot API documentation
    dfu
    efi
    linker_lists
+   pinctrl
    rng
    serial
    unicode

doc/api/pinctrl.rst

@@ -0,0 +1,7 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Pinctrl and Pinmux
+==================
+
+.. kernel-doc:: include/dm/pinctrl.h
+   :internal: