MX5: rename mx51 to mx5

Rename mx51 to mx5 in order to support more mx51
like-style SOCs such as MX53 and the followings.

Signed-off-by: Jason Liu <r64343@freescale.com>
diff --git a/boards.cfg b/boards.cfg
index 4c73680..884d9ba 100644
--- a/boards.cfg
+++ b/boards.cfg
@@ -46,8 +46,8 @@
 jadecpu		arm	arm926ejs	jadecpu		syteco		mb86r0x
 suen3		arm	arm926ejs	km_arm		keymile		kirkwood
 rd6281a		arm	arm926ejs	-		Marvell		kirkwood
-mx51evk		arm	armv7		mx51evk		freescale	mx51
-vision2		arm	armv7		vision2		ttcontrol	mx51
+mx51evk		arm	armv7		mx51evk		freescale	mx5
+vision2		arm	armv7		vision2		ttcontrol	mx5
 actux1		arm	ixp
 actux2		arm	ixp
 actux3		arm	ixp