pinctrl: forward-declare struct device

Add a dummy declaration of struct device to avoid the following warning:

In file included from include/linux/pinctrl/machine.h:15:0,
                 from arch/arm/mach-tegra/board-pinmux.h:18,
                 from arch/arm/mach-tegra/board-trimslice-pinmux.c:20:
include/linux/pinctrl/pinctrl.h:115:12: warning: 'struct device' declared inside parameter list [enabled by default]
include/linux/pinctrl/pinctrl.h:115:12: warning: its scope is only this definition or declaration, which is probably not what you want [enabled by default]

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>