imx8: scu: use dedicated MU for SPL

SPL runs in EL3 mode, except MU0_A, others are not powered on,
and could not be used. However normal U-Boot use MU1_A, so we
could not reuse the one in dts. And we could not replace the one
in dts with MU0_A, because MU0_A is reserved in secure world.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
diff --git a/arch/arm/mach-imx/imx8/Kconfig b/arch/arm/mach-imx/imx8/Kconfig
index 9671107..f76a139 100644
--- a/arch/arm/mach-imx/imx8/Kconfig
+++ b/arch/arm/mach-imx/imx8/Kconfig
@@ -3,6 +3,13 @@
 config IMX8
 	bool
 
+config MU_BASE_SPL
+	hex "MU base address used in SPL"
+	default 0x5d1b0000
+	help
+	  SPL runs in EL3 mode, it use MU0_A to communicate with SCU.
+	  So we could not reuse the one in dts which is for normal U-Boot.
+
 config IMX8QXP
 	select IMX8
 	select SUPPORT_SPL