drivers: pinctrl: Fix unused parameter warnings when compiling with -Wextra

Since the pinctrl header file can be included by user-created drivers,
an application developer including this file and only applying
-Wextra to the application source files will see many warnings.

-Wold-style-declaration warning is also emitted if 'static' is not
at the beginning of a declaration.

Signed-off-by: Pete Dietl <petedietl@gmail.com>