remoteproc: mediatek: Refactor single core check and fix retrocompatibility

remoteproc: mediatek: Refactor single core check and fix retrocompatibility

In older devicetrees we had the ChromeOS EC in a node called "cros-ec"
instead of the newer "cros-ec-rpmsg", but this driver is now checking
only for the latter, breaking compatibility with those.

Besides, we can check if the SCP is single or dual core by simply
walking through the children of the main SCP node and checking if
if there's more than one "mediatek,scp-core" compatible node.

Fixes: 1fdbf0cdde98 ("remoteproc: mediatek: Probe SCP cluster on multi-core SCP")
Reported-by: "kernelci.org bot" <bot@kernelci.org>
Tested-by: Laura Nao <laura.nao@collabora.com>
Reviewed-by: Chen-Yu Tsai <wenst@chromium.org>
Tested-by: Chen-Yu Tsai <wenst@chromium.org>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20230919092336.51007-1-angelogioacchino.delregno@collabora.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>