dt-bindings: net: dp83867: Remove binding doc from U-Boot tree

U-Boot is having DT which doesn't cover all options currently supported by
driver. DT binding is aligned with Linux kernel version available here.
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/devicetree/bindings/net/ti,dp83867.txt


Based on my talk with Grygorii Strashko better will be to remove it.

Also Linux kernel bindings are being converted to yaml that's another
reason to do it only at one place.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Grygorii Strashko <grygorii.strashko@ti.com>