phy: allow drivers to flag a PHY device as internal

libphy currently always reports a PHY as an external transceiver from
the ethtool output. This is inaccurate, because some drivers should be
able to tell that a PHY device is an internal transceiver of an Ethernet
MAC. Add a new flag (PHY_IS_INTERNAL) which can be set by PHY drivers
just like other flags, and a corresponding helper: phy_is_internal()
which can be used by networking drivers to query if a given
PHY device is internal.

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