ramips: fix SDXC function for mt76x8 one eth port devices

ramips: fix SDXC function for mt76x8 one eth port devices

There are only 5 devices in mt76x8 sub-target selected the MTK SDXC
driver package. And they are all single ethernet port routers or dev
boards:

* LinkIt Smart 7688
* Onion Omega2+
* RAVPower RP-WD009
* VoCore VoCore2
* VoCore VoCore2-Lite

For these devices, they are using the ephy p1 - p4 as the SDXC IO
pins. Therefore, these GPIO pads must be configured in "digital"
IO mode.

Signed-off-by: Shiji Yang <yangshiji66@qq.com>
Link: https://github.com/openwrt/openwrt/pull/17446
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
(cherry picked from commit f5996ae947c88461029060d74ace6a85a30dd9fd)