net: phy: Add MediaTek built-in 2.5Gb ethernet PHY driver

net: phy: Add MediaTek built-in 2.5Gb ethernet PHY driver

The MediaTek MT7987/MT7988 SoCs features a built-in 2.5Gb PHY
connected to GMAC1. The PHY supports 10/100/1000/2500 Mbps
full-duplex only.

The PHY requires one or two firmware files. Firmware for MT7988 has
already been added to upstream: mediatek/mt7988/i2p5ge-phy-pmb.bin.
MT7987 has two firmware files which will be add to upstream later:
i2p5ge-phy-pmb.bin and i2p5ge-phy-DSPBitTb.bin.

Environment variable can be set for firmware data loading:
mt7987_i2p5ge_load_pmb_firmware for i2p5ge-phy-pmb.bin
mt7987_i2p5ge_load_dspbit_firmware for i2p5ge-phy-DSPBitTb.bin
mt7988_i2p5ge_load_pmb_firmware for i2p5ge-phy-pmb.bin

This driver allows dedicated weak functions to be overridden by
board to provide the firmware data:
mt7987_i2p5ge_get_fw() for MT7987
mt7988_i2p5ge_get_fw() for MT7988

To enable the PHY, add the following not to device tree:
&eth1 {
	status = "okay";
	phy-mode = "xgmii";
	phy-handle = <&phy15>;

	phy15: ethernet-phy@15 {
		compatible = "ethernet-phy-ieee802.3-c45";
		reg = <15>;
		phy-mode = "xgmii";
	};
};

Signed-off-by: Sky Huang <SkyLake.Huang@mediatek.com>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>