arm/arm64: Move barrier instructions into separate header

Commit bfb33f0b ("sunxi: mctl_mem_matches: Add missing memory
barrier") broke compilation for the Pine64, as dram_helper.c now
includes <asm/armv7.h>, which does not compile on arm64.

Fix this by moving all barrier instructions into a separate header
file, which can easily be shared between arm and arm64.
Also extend the inline assembly to take the "sy" argument, which is
optional for ARMv7, but mandatory for v8.

This fixes compilation for 64-bit sunxi boards (Pine64).

Acked-by: Ian Campbell <ijc@hellion.org.uk>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>