dt-bindings: imx6q-pcie: Clean up duplicate clocks check

The bindings referencing this file already define these constraints for
each of the variants, so the if not: then: is redundant.

Link: https://lore.kernel.org/r/20240220161924.3871774-8-Frank.Li@nxp.com


Signed-off-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>