dt-bindings: PCI: meson: Update PCIE bindings documentation

Now that a new PHYs has been introduced for AXG SoC family, update
dt bindings documentation.

Please note that this breaks backward compatibility but as not a single
devicetree uses that yet that seems ok.

Signed-off-by: Remi Pommarel <repk@triplefau.lt>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>