dt-bindings: soc: imx: drop minItems for i.MX8MM vpu blk ctrl

minItems and maxItems are set as the same value. In such case minItems is
not necessary. So drop it.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>