xlat_tables_v2: mark 'xlat_clean_dcache_range' unused

The armclang compiler can warn if a variable is declared but
is never referenced. The '__attribute__((unused))' attribute
informs the compiler to expect an unused variable, and tells
it not to issue a warning.

This patch marks the 'xlat_clean_dcache_range' function as
"unused" to fix this armclang compiler warning.

Change-Id: I7623f61c2975a01db4d1b80554dd4f9a9e0f7eb6
Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>