dt-bindings: PCI: dwc: Add max-functions EP property

dt-bindings: PCI: dwc: Add max-functions EP property

In accordance with [1] the CX_NFUNC IP-core synthesize parameter is
responsible for the number of physical functions to support in the EP
mode. Its upper limit is 32. Let's use it to constrain the number of
PCIe functions the DW PCIe EP DT-nodes can advertise.

[1] Synopsys DesignWare Cores PCI Express Controller Databook - DWC PCIe
Endpoint, Version 5.40a, March 2019, p. 887.

Link: https://lore.kernel.org/r/20221113191301.5526-9-Sergey.Semin@baikalelectronics.ru
Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>