dt-bindings: fsl: scu: add fallback compatible string for clock

SCU clock can be used in a similar way by IMX8QXP and IMX8QM SoCs.
Let's add a "fsl,scu-clk" fallback compatible string to allow other
SoCs to reuse the common part.

Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Sascha Hauer <kernel@pengutronix.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Michael Turquette <mturquette@baylibre.com>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>