mmc: fsl_esdhc_imx: Add a compatible for i.MX51

Add a compatible for i.MX51 so that i.MX51 can use this driver
via driver model.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
diff --git a/drivers/mmc/fsl_esdhc_imx.c b/drivers/mmc/fsl_esdhc_imx.c
index e0e1326..c0e7320 100644
--- a/drivers/mmc/fsl_esdhc_imx.c
+++ b/drivers/mmc/fsl_esdhc_imx.c
@@ -1706,6 +1706,7 @@
 };
 
 static const struct udevice_id fsl_esdhc_ids[] = {
+	{ .compatible = "fsl,imx51-esdhc", },
 	{ .compatible = "fsl,imx53-esdhc", },
 	{ .compatible = "fsl,imx6ul-usdhc", },
 	{ .compatible = "fsl,imx6sx-usdhc", },