arm: support Thumb-1 with CONFIG_SYS_THUMB_BUILD

When building a Thumb-1-only target with CONFIG_SYS_THUMB_BUILD,
some files fail to build, most of the time because they include
mcr instructions, which only exist for Thumb-2.

This patch introduces a Kconfig option CONFIG_THUMB2 and uses
it to select between Thumb-2 and ARM mode for the aforementioned
files.

Signed-off-by: Albert ARIBAUD <albert.u.boot@aribaud.net>
diff --git a/arch/arm/include/asm/cache.h b/arch/arm/include/asm/cache.h
index a836e9f..1f63127 100644
--- a/arch/arm/include/asm/cache.h
+++ b/arch/arm/include/asm/cache.h
@@ -16,6 +16,9 @@
 /*
  * Invalidate L2 Cache using co-proc instruction
  */
+#ifdef CONFIG_SYS_THUMB_BUILD
+void invalidate_l2_cache(void);
+#else
 static inline void invalidate_l2_cache(void)
 {
 	unsigned int val=0;
@@ -24,6 +27,7 @@
 		: : "r" (val) : "cc");
 	isb();
 }
+#endif
 
 void l2_cache_enable(void);
 void l2_cache_disable(void);