dt-bindings: mmc: arm,pl18x: Document interrupt-names is ignored

Due to inconsistency of existing DTs regarding the content of this IP
interrupt-names DT property, document this such that interrupt-names
is not used by this IP bindings.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20221013221242.218808-1-marex@denx.de


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