fix(SME): add a missing ISB

On PEs that delay the effects of writes to system registers, enabling
SME will not take effect before it is used, causing an unhandled trap.
This patch adds the isb to prevent this.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: Iad934d3e3c4b52d09c4b8058f71ffe107bcbac1d