dt-bindings: pci: apple,pcie: Add t6020 compatible string

t6020 adds some register ranges compared to t8103, so requires
a new compatible as well as the new PHY registers.

Thanks to Mark and Rob for their helpful suggestions in updating
the binding.

Suggested-by: Mark Kettenis <mark.kettenis@xs4all.nl>
Suggested-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
[maz: added PHY registers, constraints]
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Tested-by: Janne Grunau <j@jannau.net>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Acked-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Acked-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Link: https://patch.msgid.link/20250401091713.2765724-3-maz@kernel.org