dt-bindings: display: msm: Add reg bus and rotator interconnects

Apart from the already handled data bus (MAS_MDP_Pn<->DDR), there are
other connection paths:
- a path that connects rotator block to the DDR.
- a path that needs to be handled to ensure MDSS register access
  functions properly, namely the "reg bus", a.k.a the CPU-MDSS CFG
  interconnect.

Describe these paths to allow using them in device trees and in the
driver.

[Konrad: rework for one vs two MDP paths, update examples]
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/569480/
Link: https://lore.kernel.org/r/20231125-topic-rb1_feat-v3-2-4cbb567743bb@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>