arm64: Add ARCH_HAS_SG_CHAIN

It's useful to be able to chain scatterlists together for very large
scatter allocations. Define ARCH_HAS_SG_CHAIN to let this happen.

Change-Id: Iee85a8801d08e6152a85a00826b79c9e4bd0b13f
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>