Documentation: net: phy: Add phy-is-integrated binding

Add the documentation for integrated PHY. A boolean property indicates
the PHY is integrated into the same physical package as the Ethernet
MAC. If needed, muxers should be configured to ensure the integrated
PHY is used. The absence of this property indicates the muxers should
be configured so that the external PHY is used.

Signed-off-by: David Wu <david.wu@rock-chips.com>
Signed-off-by: David S. Miller <davem@davemloft.net>