net: phy: mediatek: Re-organize MediaTek ethernet phy drivers

Re-organize MediaTek ethernet phy driver files and get ready to integrate
some common functions and add new 2.5G phy driver.
mtk-ge.c: MT7530 Gphy on MT7621 & MT7531 Gphy
mtk-ge-soc.c: Built-in Gphy on MT7981 & Built-in switch Gphy on MT7988
mtk-2p5ge.c: Planned for built-in 2.5G phy on MT7988

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: SkyLake.Huang <skylake.huang@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>