net: phy: dp83869: Update port-mirroring to read straps

The device tree may not have the property set for port mirroring
because the hardware may have it strapped. If the property is not in the
DT then check the straps and set the port mirroring bit appropriately.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>