dt-bindings: pci: apple,pcie: Add t6000 support

This new SoC is compatible with the existing driver, but the block
supports 4 downstream ports, so we need to adjust the binding to
allow that.

Reviewed-by: Mark Kettenis <kettenis@openbsd.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Hector Martin <marcan@marcan.st>