ARM: dts: imx6qdl: use MAC-address from nvmem

IMX6QDL has fuse locations specified for storing the MAC for the
built-in ethernet (Table 5-8 in Reference Manual).
Define the fuse location in ocotp and refer to them in fec-ethernet.

If the cells are not flashed, the driver behavior is unchanged, i.e.
other MAC sources will be probed and a random MAC will be used as a
last resort.

Signed-off-by: Nikolaus Voss <nikolaus.voss@haag-streit.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>