dt-bindings: firmware: qcom,scm: Document that SCM can be dma-coherent

Trogdor devices use firmware backed by TF-A instead of Qualcomm's
normal TZ. On TF-A we end up mapping memory as cacheable. Specifically,
you can see in Trogdor's TF-A code [1] in qti_sip_mem_assign() that we
call qti_mmap_add_dynamic_region() with MT_RO_DATA. This translates
down to MT_MEMORY instead of MT_NON_CACHEABLE or MT_DEVICE.

Let's allow devices like trogdor to be described properly by allowing
"dma-coherent" in the SCM node.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230616081440.v2.1.Ie79b5f0ed45739695c9970df121e11d724909157@changeid
Signed-off-by: Bjorn Andersson <andersson@kernel.org>

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