uboot-mediatek: Sync phy-mode for Xiaomi Redmi Router AX6000

uboot-mediatek: Sync phy-mode for Xiaomi Redmi Router AX6000

Commit 572ea6807053 ("uboot-mediatek: add patches for MT7988 and
builds for RFB") renamed HSGMII to 2500basex, but forgot to update
the dts of Redmi Router AX6000, makes the network unusable.
This patch makes the network usable again.

Fixes: #13724
Fixes: 572ea6807053 ("uboot-mediatek: add patches for MT7988 and builds for RFB")
Signed-off-by: Furong Xu <xfr@outlook.com>
(cherry picked from commit 03987d2d11c4954e3e8afa3d44ba6213e774c742)