mx6: soc: Add ENET2 mac address support

The i.MX6SX and i.MX6UL has two ENET controllers, add support for reading
MAC address from fuse for ENET2.

Signed-off-by: Ye Li <ye.li@nxp.com>
diff --git a/doc/README.imx6 b/doc/README.imx6
index e26ab71..7c9a4ac 100644
--- a/doc/README.imx6
+++ b/doc/README.imx6
@@ -7,7 +7,10 @@
 -----------------------------------
 
 1.1 MAC Address: It is stored in fuse bank 4, with the 32 lsbs in word 2 and the
-    16 msbs in word 3.
+    16 msbs in word 3[15:0].
+    For i.MX6SX and i.MX6UL, they have two MAC addresses. The second MAC address
+    is stored in fuse bank 4, with the 16 lsb in word 3[31:16] and the 32 msbs in 
+    word 4.
 
 Example: