net: phylink: allow RGMII/RTBI in-band status

As per RGMII specification v2.0, section 3.4.1, RGMII/RTBI has an
optional in-band status feature where the PHY's link status, speed and
duplex mode can be passed to the MAC.
Allow RGMII/RTBI to use in-band status.

Signed-off-by: Qingfang DENG <dqfext@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>