ARC: Cache: Disable IOC by default

We'd like to keep IOC HW at the same state as t is right after reset when we
start Linux kernel so there will be no re-configuration of IOC on the go.

The point is U-Boot doesn't benefit a lot from IOC as it doesn't do a
lot of DMA operations especially on multiple cores simultaneously.

At the same time re-configuration of IOC in run-time might become quite
a tricky experience because we need to make sure there're no DMA
trannsactions in flight otherwise unexpected consequencses might affect
us much later and debugging those kinds of issues will be a real
nightmare.

That said let's make our life easier a little bit.

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

arch/arc/lib/cache.c

@@ -48,6 +48,9 @@ bool slc_exists __section(".data") = false;
 bool ioc_exists __section(".data") = false;
 bool pae_exists __section(".data") = false;
 
+/* To force enable IOC set ioc_enable to 'true' */
+bool ioc_enable __section(".data") = false;
+
 void read_decode_mmu_bcr(void)
 {
 	/* TODO: should we compare mmu version from BCR and from CONFIG? */
@@ -200,7 +203,7 @@ static void read_decode_cache_bcr_arcv2(void)
 	} cbcr;
 
 	cbcr.word = read_aux_reg(ARC_BCR_CLUSTER);
-	if (cbcr.fields.c)
+	if (cbcr.fields.c && ioc_enable)
 		ioc_exists = true;
 }
 #endif