mx28evk: add trimffs to nand command

this is usefull when writing an UBI image which contains
and UBIFS volume (check README.nand and UBI FAQ for more details)

Signed-off-by: Eric Bénard <eric@eukrea.com>
Acked-by: Otavio Salvador <otavio@ossystems.com.br>

include/configs/mx28evk.h

@@ -63,6 +63,7 @@
 #define CONFIG_CMD_USB
 #define CONFIG_CMD_BOOTZ
 #define CONFIG_CMD_NAND
+#define CONFIG_CMD_NAND_TRIMFFS
 
 /* Memory configurations */
 #define CONFIG_NR_DRAM_BANKS		1		/* 1 bank of DRAM */