arm64: Enable handling of CP15 barrier operations

Some legacy userspace application may rely on CP15 barrier
instructions to handle barrier operations. Set the CP15BEN
bit in SCTLR_EL1 to enable support for these instructions.

Change-Id: I293e3477b65d96f09ec5a289b73502b6a667307c
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>