net: xgene: Move shared header file into include/linux

[ Upstream commit 232e15e1

 ]

This header file is currently included into the ethernet driver via a
relative path into the PHY subsystem. This is bad practice, and causes
issues for the upcoming move of the MDIO driver. Move the header file
into include/linux to clean this up.

v2:
Move header to include/linux/mdio

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: 7083df59

 ("net: mdio-mux-meson-g12a: force internal PHY off on mux switch")
Signed-off-by: Sasha Levin <sashal@kernel.org>