dt-bindings: mmc: Fix reference to pwr-seq-simple

It's a YAML file nowadays.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Link: https://lore.kernel.org/r/d7a0be6a2688a9829077cc21ca4a5bf9528f9eb1.1691606520.git.agx@sigxcpu.org


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