gpio: tegra: Hide tegra_gpio_enable/disable()

Recent pinctrl discussions concluded that gpiolib APIs should in fact do
whatever is required to mux a GPIO onto pins, by calling pinctrl APIs if
required. This change implements this for the Tegra GPIO driver, and removes
calls to the Tegra-specific APIs from drivers and board files.

Cc: Chris Ball <cjb@laptop.org>
Cc: linux-mmc@vger.kernel.org
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Chris Ball <cjb@laptop.org> # for sdhci-tegra.c
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Olof Johansson <olof@lixom.net>