dt-bindings: soc: smem: Make indirection optional

In the olden days the Qualcomm shared memory (SMEM) region consisted of
multiple chunks of memory, so SMEM was described as a standalone node
with references to its various memory regions.

But practically all modern Qualcomm platforms has a single reserved memory
region used for SMEM. So rather than having to use two nodes to describe
the one SMEM region, update the binding to allow the reserved-memory
region alone to describe SMEM.

The olden format is preserved as valid, as this is widely used already.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Vladimir Zapolskiy <vladimir.zapolskiy@linaro.org>
Link: https://lore.kernel.org/r/20210930182111.57353-3-bjorn.andersson@linaro.org