dt-bindings: sram: convert rockchip-pmu-sram bindings to yaml

Current dts files with 'rockchip-pmu-sram' compatible nodes
are now verified with sram.yaml, although the original
text document still exists. Merge rockchip-pmu-sram.txt
with sram.yaml by adding it as description with an example.
Make #address-cells, #size-cells and ranges optional
if there are no child nodes to prevent yaml warnings.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>