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/lib/start.S b/arch/arc/lib/start.S
index 48ee86e..e1ef19c 100644
--- a/arch/arc/lib/start.S
+++ b/arch/arc/lib/start.S
@@ -18,6 +18,10 @@
 	mov	%fp, %sp
 
 	/* Unconditionally disable caches */
+#ifdef CONFIG_ISA_ARCV2
+	bl	slc_flush
+	bl	slc_disable
+#endif
 	bl	flush_dcache_all
 	bl	dcache_disable
 	bl	icache_disable