net: phy: dp83867: Add device tree bindings and documentation

Add the device tree bindings and the accompanying documentation
for the TI DP83867 Giga bit ethernet phy driver.

The original document was from:
    [commit 2a10154abcb75ad0d7b6bfea6210ac743ec60897 from the Linux kernel]

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Reviewed-by: Mugunthan V N <mugunthanvnm@ti.com>
Tested-by: Mugunthan V N <mugunthanvnm@ti.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>