ARM: dts: imx27: Adjust the GPT compatible string

include/soc/imx/timer.h describes well the different versions of
the GPT block among the imx family:

enum imx_gpt_type {
	GPT_TYPE_IMX1,		/* i.MX1 */
	GPT_TYPE_IMX21,		/* i.MX21/27 */
	GPT_TYPE_IMX31,		/* i.MX31/35/25/37/51/6Q */
	GPT_TYPE_IMX6DL,	/* i.MX6DL/SX/SL */
};

So the proper compatible string for the MX27 case should be
"fsl,imx21-gpt".

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Reviewed-by: Philippe Reynes <tremyfr@gmail.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>