arm64: dts: mediatek: Add MT8186 Magneton Chromebooks

Add entries for the MT8186 based Chromebooks, also collectively known
as the Lenovo IdeaPad Slim 3 Chromebook (14M868). It is also based on
the "Steelix" design. Being a laptop instead of a convertible device,
there is no stylus, which is similar to Rusty. However Magneton does
not have ports on the right side of the device.

Three variants are listed separately. These use different touchscreen
controllers, or lack a touchscreen altogether.

Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20240126083802.2728610-10-wenst@chromium.org
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>