firmware: imx: add COMPILE_TEST for IMX_SCU driver

Add COMPILE_TEST support to IMX_SCU driver for better compile
testing coverage.  Any driver depending on IMX_SCU shouldn't have
COMPILE_TEST though.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>