dt-bindings: PCI: Cleanup MediaTek binding text

To accommodate other SoC generations, regroup specific properties by SoC,
and remove redundant descriptions.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Honghui Zhang <honghui.zhang@mediatek.com>
[bhelgaas: split into a rename patch and a cleanup patch]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rob Herring <robh@kernel.org>