dt-bindings: bus: simple-pm-bus: convert bindings to json-schema

Convert Simple Power-Managed Bus bindings documentation to json-schema.

As a side effect of this change only simple-pm-bus is used in example. A
follow-up patch will provide an example for the separately documented
Renesas Bus State Controller (BSC) that uses "renesas,bsc-sh73a0" and
"renesas,bsc" compat strings.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Rob Herring <robh@kernel.org>