dt-bindings: PCI: dwc: Apply generic schema for generic device only

dt-bindings: PCI: dwc: Apply generic schema for generic device only

Having the generic compatible strings constraints with the 'any'+'generic
string' semantic implicitly encourages either to add new DW PCIe-based
DT-bindings with the generic compatible string attached or just forget
about adding new DT-bindings since the corresponding DT-node will be
evaluated anyway. Moreover having that semantic implemented in the
generic DT-schema causes the DT-validation tool to apply the schema twice:
first by implicit compatible-string-based selection and second by means of
the 'allOf: [ $ref ]' statement. Let's fix all of that by dropping the
compatible property constraints and selecting the generic DT-schema only
for the purely generic DW PCIe DT-nodes. The later is required since there
is a driver for such devices. (Though there are no such DT-nodes currently
defined in the kernel DT sources.)

Link: https://lore.kernel.org/r/20221113191301.5526-8-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>