imx: Don't define __ASSEMBLY__ in source files

This is supposed to be a build-system flag. Move it there so we can
define it before linux/kconfig.h is included.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/arm/mach-imx/Makefile b/arch/arm/mach-imx/Makefile
index 07954bc..77e7270 100644
--- a/arch/arm/mach-imx/Makefile
+++ b/arch/arm/mach-imx/Makefile
@@ -91,7 +91,7 @@
 endif
 
 quiet_cmd_cpp_cfg = CFGS    $@
-      cmd_cpp_cfg = $(CPP) $(cpp_flags) -x c -o $@ $<
+      cmd_cpp_cfg = $(CPP) $(cpp_flags) -D__ASSEMBLY__ -x c -o $@ $<
 
 # mkimage source config file
 IMX_CONFIG = $(CONFIG_IMX_CONFIG:"%"=%)