dt-bindings: PCI: qcom-ep: Describe optional IOMMU

dt-bindings: PCI: qcom-ep: Describe optional IOMMU

Some of Qualcomm platforms have an IOMMU unit between the PCIe IP and
DDR. For example, the SA8775P specifies the iommu alththough it is not
a part of bindings.

Thus, change the schema in order to require the IOMMU for SA8775P and
forbid it from being used on SDX55 (SM8450 will be handled in a later
patch).

This fixes the following warning:

  pcie-ep@1c10000: Unevaluated properties are not allowed ('iommus' was unexpected)

Fixes: 9d3d5e75f31c ("dt-bindings: PCI: qcom-ep: Add support for SA8775P SoC")
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Link: https://lore.kernel.org/r/20250221-sar2130p-pci-v3-2-61a0fdfb75b4@linaro.org
[kwilczynski: commit log]
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>