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>