dt-bindings: serial: Allow embedded-controller as child node

There exist some embedded controllers (like Microsoft SAM found on
Surface devices or Apple Oscar found on old iPhones) that connect to
the host device via serial.

Allow that class of devices to exist under serial interface controller
nodes.

Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Konrad Dybcio <quic_kdybcio@quicinc.com>
Link: https://lore.kernel.org/r/20240814-topic-sam-v3-1-a84588aad233@quicinc.com


Signed-off-by: Hans de Goede <hdegoede@redhat.com>