dt-bindings: mmc: fsl-imx-mmc: Document the required clocks

The fsl-imx-mmc hardware needs two clocks to operate: ipg and per.

Document these required clocks.

This fixes the following schema warning:

imx27-apf27dev.dtb: mmc@10014000: Unevaluated properties are not allowed ('clock-names', 'clocks' were unexpected)
	from schema $id: http://devicetree.org/schemas/mmc/fsl-imx-mmc.yaml#



Signed-off-by: Fabio Estevam <festevam@denx.de>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240222143911.979058-1-festevam@gmail.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>