dt-bindings: PCI: Make "cdns,max-outbound-regions" optional property

Make "cdns,max-outbound-regions" optional property with the default
being 32.

Link: http://lore.kernel.org/r/20201105165331.GA55814@bogus
Link: https://lore.kernel.org/r/20201106151107.3987-2-kishon@ti.com


Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>