arm64: rename __asm_flush_dcache_level to __asm_dcache_level

Since 1e6ad55c ("armv8/cache: Change cache invalidate and flush
function"), this routine can be used for both cache flushing and
cache invalidation.  So, it is better to not include "flush" in
this routine name.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>