dt-bindings: PCI: qcom,pcie-ipq9574: Move IPQ9574 to dedicated schema

dt-bindings: PCI: qcom,pcie-ipq9574: Move IPQ9574 to dedicated schema

Move IPQ9574 and compatible PCIe devices from qcom,pcie.yaml binding to
a dedicated file to make reviewing and maintenance easier.

New schema is equivalent to the old one with few changes:
 - Adding a required compatible, which is actually redundant.
 - Drop the really obvious comments next to clock/reg/reset-names items.
 - Make last "reg" entry "mhi" a required one, because all in-tree DTS
   were updated to include it.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/20251217-dt-bindings-pci-qcom-v2-9-873721599754@oss.qualcomm.com