rampis: mt7621: Convert Edgerouter-X dts to single kernel config

rampis: mt7621: Convert Edgerouter-X dts to single kernel config

The OEM layout for Edgerouter X provides for two 3MB kernel slots.
As of linux 6.1 the kernel images no longer fit and as such
Edgerouter X builds have been disabled in Main.

Revise the layout to make kernel1 slot 6MB and drop kernel2 slot.

This patch applies the required changes to the dts file.

Signed-off-by: Tim Lunn <tim@feathertop.org>
Tested-by: Mauri Sandberg <maukka@ext.kapsi.fi>
Link: https://github.com/openwrt/openwrt/pull/15194
Signed-off-by: Petr Štetiar <ynezz@true.cz>