ARM: i.MX51: Add GPT devicetree node

The GPT is the GPT timer found on i.MX SoCs. This patch adds the
devicetree node for it.

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