imx: fec: add MAC reading from eFuses to README

Extend the documentation of the fec_mxc configuration by describing its
ability to read the ethaddr MAC address from the SoC eFuses.

Also add an example how to program the fuses for an imx5 to clarify the
byte order.

Cc: Stefano Babic <sbabic at denx.de>
Cc: Marek Vasut <marex at denx.de>
Signed-off-by: Olaf Mandel <o.mandel at menlosystems.com>

doc/README.fec_mxc

 U-boot config options used in fec_mxc.c
 
 CONFIG_FEC_MXC
-	Selects fec_mxc.c to be compiled into u-boot.
+	Selects fec_mxc.c to be compiled into u-boot. Can read out the
+	ethaddr from the SoC eFuses (see below).
 
 CONFIG_MII
 	Must be defined if CONFIG_FEC_MXC is defined.
@@ -25,3 +26,9 @@ CONFIG_FEC_MXC_NO_ANEG
 CONFIG_FEC_MXC_PHYADDR
 	Optional, selects the exact phy address that should be connected
 	and function fecmxc_initialize will try to initialize it.
+
+
+Reading the ethaddr from the SoC eFuses:
+if CONFIG_FEC_MXC is defined and the U-Boot environment does not contain the
+ethaddr variable, then its value gets read from the corresponding eFuses in
+the SoC. See the README files of the specific SoC for details.

doc/README.imx5

@@ -26,3 +26,15 @@ i.MX5x SoCs.
 
 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).
+
+    This is an example how to program an example MAC address 01:23:45:67:89:ab
+    into the eFuses. Assure that the programming voltage is available and then
+    execute:
+
+    => fuse prog -y 1 9 01 23 45 67 89 ab
+
+    After programming a MAC address, consider locking the MAC fuses. This is
+    done by programming the MAC_ADDR_LOCK fuse, which is bit 4 of word 0 in
+    bank 1:
+
+    => fuse prog -y 1 0 10