net: phy: mediatek: Move LED helper functions into mtk phy lib

This patch creates mtk-phy-lib.c & mtk-phy.h and integrates mtk-ge-soc.c's
LED helper functions so that we can use those helper functions in other
MTK's ethernet phy driver.

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>