dt-bindings: power: qcom,rpmpd: document the Shikra RPM Power Domains

Document the RPM Power Domains on the Shikra Platform.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Signed-off-by: Rakesh Kota <rakesh.kota@oss.qualcomm.com>
Signed-off-by: Ulf Hansson <ulfh@kernel.org>