dt-bindings: PCI: aardvark: Describe new properties

dt-bindings: PCI: aardvark: Describe new properties

Document the possibility to reference a PHY and reset-gpios and to set
max-link-speed property.

Link: https://lore.kernel.org/r/20200430080625.26070-10-pali@kernel.org
Tested-by: Tomasz Maciej Nowak <tmn505@gmail.com>
Signed-off-by: Marek Behún <marek.behun@nic.cz>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: devicetree@vger.kernel.org