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/lib/init_helpers.c

@@ -10,16 +10,8 @@ DECLARE_GLOBAL_DATA_PTR;
 
 int init_cache_f_r(void)
 {
-#ifndef CONFIG_SYS_ICACHE_OFF
-	icache_enable();
-	/* Make sure no stale entries persist from before we disabled cache */
-	invalidate_icache_all();
-#endif
-
 #ifndef CONFIG_SYS_DCACHE_OFF
-	dcache_enable();
-	/* Make sure no stale entries persist from before we disabled cache */
-	invalidate_dcache_all();
+	flush_dcache_all();
 #endif
 	return 0;
 }