mx25pdk: Include CONFIG_MX25

It is necessary to include CONFIG_MX25 as several i.mx drivers handle the SoC
differences based on the this config option.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>

include/configs/mx25pdk.h

@@ -19,6 +19,7 @@
 
 /* High Level Configuration Options */
 
+#define CONFIG_MX25
 #define CONFIG_SYS_HZ			1000
 #define CONFIG_SYS_TEXT_BASE		0x81200000