ARM: ep93xx: move pinctrl interfaces into include/linux/soc

ep93xx does not have a proper pinctrl driver, but does things
ad-hoc through mach/platform.h, which is also used for setting
up the boards.

To avoid using mach/*.h headers completely, let's move the interfaces
into include/linux/soc/. This is far from great, but gets the job
done here, without the need for a proper pinctrl driver.

Acked-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Acked-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Olof Johansson <olof@lixom.net>