locks: bakery: use is_dcache_enabled() helper

bakery_lock_normal.c uses the raw register accessor, read_sctlr(_el3)
to check whether the dcache is enabled.

Using is_dcache_enabled() is cleaner, and a good abstraction for
the library code like this.

A problem is is_dcache_enabled() is declared in the local header,
lib/xlat_tables_v2/xlat_tables_private.h

I searched for a good place to declare this helper. Moving it to
arch_helpers.h, closed to cache operation helpers, looks good enough
to me.

I also changed the type of 'is_cached' to bool for consistency,
and to avoid MISRA warnings.

Change-Id: I9b016f67bc8eade25c316aa9c0db0fa4cd375b79
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/lib/xlat_tables_v2/xlat_tables_private.h b/lib/xlat_tables_v2/xlat_tables_private.h
index 70ef395..863470c 100644
--- a/lib/xlat_tables_v2/xlat_tables_private.h
+++ b/lib/xlat_tables_v2/xlat_tables_private.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2017-2018, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2017-2020, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -99,9 +99,6 @@
  */
 bool is_mmu_enabled_ctx(const xlat_ctx_t *ctx);
 
-/* Returns true if the data cache is enabled at the current EL. */
-bool is_dcache_enabled(void);
-
 /*
  * Returns minimum virtual address space size supported by the architecture
  */