arm64: Enable support for the SETEND instruction

Since some legacy applications might use the SETEND
instruction, enable support for this instruction by
clearing the SCTLR[SED] bit on startup.

Change-Id: Ie10e38cbf5f27c379ab8be0f000a6bcd2fae6e9d
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>