dt-bindings: PCI: mediatek-gen3: add support for mt7986

Add compatible string and clock-definition for mt7986. It needs 4 clocks
for PCIe, define them in binding.

Link: https://lore.kernel.org/r/20221127114142.156573-5-linux@fw-web.de


Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Acked-by: Jianjun Wang <jianjun.wang@mediatek.com>