ramips: dts: adjust the power regulators of MMC controller

ramips: dts: adjust the power regulators of MMC controller

- Fix MMC bus IO voltage. The GPIO voltage of mt762x series SoCs
  is fixed 3.3V. There are no visible registers that can control
  the voltage level. Also add "no-1-8-v" property to indicate that
  MMC controller doesn't support 1.8V IO.

- Drop useless property "enable-active-high". These dummy fixed
  voltage regulators are not controlled by the GPIO. We don't need
  to set the GPIO polarity.

Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Link: https://github.com/openwrt/openwrt/pull/18886
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>