pinctrl: tegra: define GPIO compatible node per SoC

Tegra 2 uses a different GPIO controller which uses "tegra20-gpio" as
compatible string.

Make the compatible string the GPIO node is using a SoC specific
property. This prevents the kernel from registering the GPIO range
twice in case the GPIO range is specified in the device tree.

Fixes: 9462510c ("pinctrl: tegra: Only set the gpio range if needed")
Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>