arm64: a backwards compatible config option

Create a config option that when selected configures the kernel to be
as backwards compatable with kernels that ran on an ARMv7 processor
as possible.

Change-Id: I7cd67e6d4174335f9a67aba2a39dfd993f240c27
Signed-off-by: Alex Van Brunt <avanbrunt@nvidia.com>
Reviewed-on: http://git-master/r/366094


Reviewed-by: Richard Wiley <rwiley@nvidia.com>
Reviewed-by: Automatic_Commit_Validation_User
Tested-by: Oskari Jaaskelainen <oskarij@nvidia.com>