dt-bindings: Convert Reserved Memory binding to a schema

Convert the reserved memory bindings to DT schema format. The bindings
are split into schemas for clients, common reserved memory node
properties and a specific binding for shared-dma-pool.

Cc: devicetree-spec@vger.kernel.org
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210901091852.479202-6-maxime@cerno.tech