ramips: disable sdhc for HC5661A

ramips: disable sdhc for HC5661A

Currently OpenWrt doesn't support switching MT7628 into AP mode
(which is done by writing some undocumented registers in MTK SDK)
Without doing so, enabling SD breaks 4 FE ports and the SD controller
doesn't work since SD pins aren't configured correctly.

Disable SDHC on HC5661A to recover the 4 FE ports.

Signed-off-by: Chuanhong Guo <gch981213@gmail.com>
[drop the sdhci node completely]
Signed-off-by: Mathias Kresin <dev@kresin.me>
(backported from 5563d49a391e65fd9c179661b9ddfd79cb903a54)