net: phy: add Maxlinear GPY115/21x/24x driver

Add driver to support the Maxlinear GPY115, GPY211, GPY212, GPY215,
GPY241, GPY245 PHYs. Separate from XWAY PHY driver because this series
has different register layout and new features not supported in XWAY PHY.

Signed-off-by: Xu Liang <lxu@maxlinear.com>
Acked-by: Hauke Mehrtens <hmehrtens@maxlinear.com>
Tested-by: Wong Vee Khee <vee.khee.wong@linux.intel.com>
Tested-by: Wong Vee Khee <vee.khee.wong@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>