In my box, for sdio it was enough to reduce the clock to 40MHz.
Here is the link to my changes: MECOOL M8S PLUS (new, s905x2)
Check if sdio is working now:
CoreELEC:~ # cat /sys/kernel/debug/sdio/ios
clock: 40000000 Hz
actual clock: 40000000 Hz
vdd: 21 (3.3 ~ 3.4 V)
bus mode: 2 (push-pull)
chip select: 0 (don't care)
power mode: 1 (up)
bus width: 2 (4 bits)
timing spec: 2 (sd high-speed)
signal voltage: 0 (3.30 V)
driver type: 0 (driver type B)
CoreELEC:~ #