dt-bindings: mmc: mmc-pwreq-simple: Accept more than one reset GPIO

dt-bindings: mmc: mmc-pwreq-simple: Accept more than one reset GPIO

There might be multiple reset GPIOs but dtschema has trouble parsing it
if there are no maxItems:

  arch/arm/boot/dts/exynos5250-snow.dt.yaml: mmc3_pwrseq: reset-gpios: [[20, 2, 1], [20, 1, 1]] is too long
    From schema: Documentation/devicetree/bindings/mmc/mmc-pwrseq-simple.yaml

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200831161147.13515-2-krzk@kernel.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>