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>

drivers/firmware/imx/Kconfig

@@ -12,7 +12,7 @@ config IMX_DSP
 
 config IMX_SCU
 	bool "IMX SCU Protocol driver"
-	depends on IMX_MBOX
+	depends on IMX_MBOX || COMPILE_TEST
 	help
 	  The System Controller Firmware (SCFW) is a low-level system function
 	  which runs on a dedicated Cortex-M core to provide power, clock, and