dt-bindings: soc: imx8mp-hsio-blk-ctrl: add clock cells

The HSIO blk-ctrl has a internal PLL, which can be used as a reference
clock for the PCIe PHY. Add clock-cells to the binding to allow the
driver to expose this PLL.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>