dt-bindings: interconnect: qcom: document the interconnects for sa8775p

Add a DT binding document for the RPMh interconnects on Qualcomm sa8775p
platforms.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230118140825.242544-2-brgl@bgdev.pl
Signed-off-by: Georgi Djakov <djakov@kernel.org>