clk: imx: scu: add fallback compatible string support

SCU clock can be used in a similar way by IMX8QXP and IMX8QM SoCs.
Make the driver support "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: Stephen Boyd <sboyd@kernel.org>
Cc: Michael Turquette <mturquette@baylibre.com>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>