net: phy: add driver for Motorcomm yt8511 phy

Add a driver for the Motorcomm yt8511 phy that will be used in the
production Pine64 rk3566-quartz64 development board.
It supports gigabit transfer speeds, rgmii, and 125mhz clk output.

Signed-off-by: Peter Geis <pgwipeout@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>