add arm64 cache flushing code from linux v3.16

add arm64 cache flushing code from linux v3.16

__flush_dcache_all added from arch/arm64/mm/cache.S, with helper macros from
arch/arm64/include/asm/assembler.h, from v3.16.  The cache flushing is required
when transitioning from EFI code that runs with cache enable to Xen startup
code which expects the cache to be disabled.

Signed-off-by: Roy Franz <roy.franz@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- removed indent on ENTRY() and dropped the entry point label which
         duplicates the one from the macro. ]