PCI: Add DT binding for "reset-gpios" property

Add DT binding for "reset-gpios" property which supports GPIO based PERST#
signal.

Signed-off-by: Manikanta Maddireddy <[e-mail]>
Signed-off-by: Lorenzo Pieralisi <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Acked-by: Thierry Reding <[e-mail]>

Documentation/devicetree/bindings/pci/pci.txt

···
    unsupported link speed, for instance, trying to do training for
    unsupported link speed, etc.  Must be '4' for gen4, '3' for gen3, '2'
    for gen2, and '1' for gen1. Any other values are invalid.
+- reset-gpios:
+   If present this property specifies PERST# GPIO. Host drivers can parse the
+   GPIO and apply fundamental reset to endpoints.
 
 PCI-PCI Bridge properties
 -------------------------