dt-bindings: PCI: snps,dw-pcie-ep: Drop conflicting 'max-functions' schema

'max-functions' is already defined in pci-ep.yaml schema as a uint8 and all
users of it expect an uint8. Drop the conflicting schema.

Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220107030358.2378221-1-robh@kernel.org