dt-bindings: soc: imx8m-blk-ctrl: Rename blk_ctrl to blk-ctrl in examples

Rename blk_ctrl@ to blk-ctrl (with dash instead of underscore) in examples
so that they would match the imx8m[mn].dtsi in current Linux kernel.

Signed-off-by: Marek Vasut <marex@denx.de>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>