dt-bindings: PCI: Rename MediaTek binding

To accommodate other SoC generations, rename mediatek,mt7623-pcie.txt to
mediatek-pcie.txt.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Honghui Zhang <honghui.zhang@mediatek.com>
[bhelgaas: split rename to separate patch so updates are obvious]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rob Herring <robh@kernel.org>