dt-bindings: phy: mediatek,mt7621-pci-phy: add clock entries

MT7621 SoC clock driver has already mainlined in
'commit 48df7a26

 ("clk: ralink: add clock driver for mt7621 SoC")'
Hence update schema with the add of the entries related to
clock. Since until now things were not properly being done
we mark also 'clock' as required in the binding since this
will be now the only way to properly retrieve frequency to be
able to make a correct configuration of the PCIe phy registers.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210508070930.5290-3-sergio.paracuellos@gmail.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>