defconfig: r4: add mtk 2g5 phy driver

https://git.kernel.org/pub/scm/linux/kernel/git/firmware/linux-firmware.git/tree/mediatek/mt7988/i2p5ge-phy-pmb.bin

root@bpi-r4-8G:~# modprobe mtk-2p5ge
root@bpi-r4-8G:~# ip link set eth1 up
[  182.269515] MediaTek MT7988 2.5GbE PHY mdio-bus:0f: Firmware date code: 0/0/0, version: 0.0
[  182.285470] MediaTek MT7988 2.5GbE PHY mdio-bus:0f: Firmware loading/trigger ok.
[  182.293746] mtk_soc_eth 15100000.ethernet eth1: PHY [mdio-bus:0f] driver [MediaTek MT7988 2.5GbE PHY] (irq=POLL)
[  182.304245] mtk_soc_eth 15100000.ethernet eth1: configuring for phy/internal link mode
root@bpi-r4-8G:~# [  186.455774] mtk_soc_eth 15100000.ethernet eth1: Link is Up - 1Gbps/Full - flow control rx/tx

Author: frank-w

arch/arm64/configs/mt7988a_bpi-r4_defconfig

@@ -1298,7 +1298,7 @@ CONFIG_MTK_T7XX=m
 
 CONFIG_MARVELL_PHY=m
 CONFIG_MARVELL_10G_PHY=m
-
+CONFIG_MEDIATEK_2P5GE_PHY=m
 #spi display
 CONFIG_DRM=m
 CONFIG_FB=m