arc: significant cache rework

[1] Align cache management functions to those in Linux kernel. I.e.:
    a) Use the same functions for all cache ops (D$ Inv/Flush)
    b) Split cache ops in 3 sub-functions: "before", "lineloop" and
"after". That way we may re-use "before" and "after" functions for
region and full cache ops.

 [2] Implement full-functional L2 (SLC) management. Before SLC was
simply disabled early on boot. It's also possible to enable or disable
L2 cache from config utility.

 [3] Disable/enable corresponding caches early on boot. So if U-Boot is
configured to use caches they will be used at all times (this is useful
in partucular for speed-up of relocation).

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>

arch/arc/include/asm/cache.h

@@ -29,12 +29,7 @@
 
 #ifndef __ASSEMBLY__
 
-#ifdef CONFIG_ISA_ARCV2
-void slc_enable(void);
-void slc_disable(void);
-void slc_flush(void);
-void slc_invalidate(void);
-#endif
+void cache_init(void);
 
 #endif /* __ASSEMBLY__ */