net: phy: dp83867: Add lane swapping support in the DP83867 TI's PHY driver

This patch adds support for enabling or disabling the lane swapping (called
"port mirroring" in PHY's CFG4 register) feature of the DP83867 TI's PHY
device.

One use case is when bootstrap configuration enables this feature (because
of e.g. LED_0 wrong wiring) so then one needs to disable it in software
(at u-boot/Linux).

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>