imx8: save args passed from ATF

We use information from ATF to know whether OP-TEE is running or not.
So save args passed from ATF.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
diff --git a/arch/arm/include/asm/arch-imx8/sys_proto.h b/arch/arm/include/asm/arch-imx8/sys_proto.h
index b8d2a0b..0e981ae 100644
--- a/arch/arm/include/asm/arch-imx8/sys_proto.h
+++ b/arch/arm/include/asm/arch-imx8/sys_proto.h
@@ -16,6 +16,7 @@
 	u32 g_ap_mu;
 };
 
+extern unsigned long boot_pointer[];
 void build_info(void);
 enum boot_device get_boot_device(void);
 int print_bootinfo(void);