arm64: optionally set CP15BEN in SCTLR

Setting CP15BEN allows legacy applications running in AArch32 mode
that use CP15 DMB as similar instructions to continue running.

Change-Id: If76d3c6ee12865ff8c4b4e7aed01146bead87773
Signed-off-by: Alex Van Brunt <avanbrunt@nvidia.com>
Reviewed-on: http://git-master/r/366096


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