octeon: ubnt-*: Add device specific DTS pruning

octeon: ubnt-*: Add device specific DTS pruning

This imports device specific DTS pruning from
https://github.com/UI-Packages/kernel_e200/blob/master/arch/mips/cavium-octeon/octeon-platform.c#L1067

- Reduce MMC clock frequency on E200/E220 to make
  MMC communication reliable again. See linked issue.
- Remove unused MMC node on E300.

Link: https://github.com/openwrt/openwrt/issues/13762
Signed-off-by: Jakob Haufe <sur5r@sur5r.net>
Link: https://github.com/openwrt/openwrt/pull/19229
Signed-off-by: Robert Marko <robimarko@gmail.com>
(cherry-picked from commit 6e58e645018ec6d429cc410bf81fce0f147c9885)
Link: https://github.com/openwrt/openwrt/pull/19374
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>