arm64: dts: imx8mp: add clock-cells to hsio-blk-ctrl

The HSIO blk-ctrl exposes the high performance PLL as
a simple clock.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Tested-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Tested-by: Lukas F. Hartmann <lukas@mntre.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>