imx: Document fuse assignments for MAC addresses

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>

doc/README.imx25

+U-Boot for Freescale i.MX25
+
+This file contains information for the port of U-Boot to the Freescale i.MX25
+SoC.
+
+1. CONVENTIONS FOR FUSE ASSIGNMENTS
+-----------------------------------
+
+1.1 MAC Address: It is stored in the words 26 to 31 of fuse bank 0, using the
+    natural MAC byte order (i.e. MSB first).

doc/README.imx27

+U-Boot for Freescale i.MX27
+
+This file contains information for the port of U-Boot to the Freescale i.MX27
+SoC.
+
+1. CONVENTIONS FOR FUSE ASSIGNMENTS
+-----------------------------------
+
+1.1 MAC Address: It is stored in the words 4 to 9 of fuse bank 0, using the
+    reversed MAC byte order (i.e. LSB first).

doc/README.imx5

@@ -20,3 +20,9 @@ i.MX5x SoCs.
     This option should be enabled for boards having a SYS_ON_OFF_CTL signal
     connected to GPIO1[23] and triggering the MAIN_PWR_ON signal like in the
     reference designs.
+
+2. CONVENTIONS FOR FUSE ASSIGNMENTS
+-----------------------------------
+
+2.1 MAC Address: It is stored in the words 9 to 14 of fuse bank 1, using the
+    natural MAC byte order (i.e. MSB first).

doc/README.imx6

+U-Boot for Freescale i.MX6
+
+This file contains information for the port of U-Boot to the Freescale i.MX6
+SoC.
+
+1. CONVENTIONS FOR FUSE ASSIGNMENTS
+-----------------------------------
+
+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.