ARM: dts: imx: add alias for gpio

Add alias for gpio nodes, so that gpio driver can identify the port
number and then specify a sensible gpio base rather than using the
one dynamically allocated by gpio core.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>