dt-bindings: PCI: qcom: Add "mhi" register region to supported SoCs

"mhi" register region contains the MHI registers that could be used by
the PCIe controller drivers to get debug information like PCIe link
transition counts on newer SoCs.

Link: https://lore.kernel.org/r/20230316081117.14288-16-manivannan.sadhasivam@linaro.org


Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>