arm64: dts: qcom: sc8280xp: Add "mhi" region to the PCIe nodes

The "mhi" region contains the debug registers that could be used to monitor
the PCIe link transitions.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230316081117.14288-19-manivannan.sadhasivam@linaro.org