arc: add support for SLC (System Level Cache, AKA L2-cache)

ARCv2 cores may have built-in SLC (System Level Cache, AKA L2-cache).
This change adds functions required for controlling SLC:
 * slc_enable/disable
 * slc_flush/invalidate

For now we just disable SLC to escape DMA coherency issues until either:
 * SLC flush/invalidate is supported in DMA APIin U-Boot
 * hardware DMA coherency is implemented (that might be board specific
   so probably we'll need to have a separate Kconfig option for
   controlling SLC explicitly)

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
diff --git a/arch/arc/include/asm/cache.h b/arch/arc/include/asm/cache.h
index 8a77cd9..0b3ebd9 100644
--- a/arch/arc/include/asm/cache.h
+++ b/arch/arc/include/asm/cache.h
@@ -27,4 +27,15 @@
 #define CONFIG_ARC_MMU_VER 4
 #endif
 
+#ifndef __ASSEMBLY__
+
+#ifdef CONFIG_ISA_ARCV2
+void slc_enable(void);
+void slc_disable(void);
+void slc_flush(void);
+void slc_invalidate(void);
+#endif
+
+#endif /* __ASSEMBLY__ */
+
 #endif /* __ASM_ARC_CACHE_H */