ARM: i.MX: Add GPT devicetree Documentation

The GPT binding is already used on i.MX6 and i.MX25, but not yet
documented. Add a binding document for it.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>