dt-bindings: PCI: fu740-pci: fix missing clock-names

The commit b92225b0 ("dt-bindings: PCI: designware: Fix
'unevaluatedProperties' warnings") removed the clock-names property as
a requirement and from the example as it triggered unevaluatedProperty
warnings. dtbs_check was not able to pick up on this at the time, but
now can:

arch/riscv/boot/dts/sifive/hifive-unmatched-a00.dtb: pcie@e00000000: Unevaluated properties are not allowed ('clock-names' was unexpected)
        From schema: linux/Documentation/devicetree/bindings/pci/sifive,fu740-pcie.yaml

The property was already in use by the FU740 DTS and the clock must be
enabled. The Linux and FreeBSD drivers require the property to enable
the clocks correctly Re-add the property and its "clocks" dependency,
while making it required.

Link: https://lore.kernel.org/r/20220819231415.3860210-2-mail@conchuod.ie


Fixes: b92225b0 ("dt-bindings: PCI: designware: Fix 'unevaluatedProperties' warnings")
Fixes: 43cea116 ("dt-bindings: PCI: Add SiFive FU740 PCIe host controller")
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>