ARM: i.MX31: Add GPT devicetree node

The GPT is the GPT timer found on i.MX SoCs.

Since this is the first user of the AIPS2 this patch also adds it.

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