fix(security): optimisations for CVE-2022-23960

Optimised the loop workaround for Spectre_BHB mitigation:
1. use of speculation barrier for cores implementing SB instruction.
2. use str/ldr instead of stp/ldp as the loop uses only X2 register.

Signed-off-by: Bipin Ravi <bipin.ravi@arm.com>
Change-Id: I8ac53ea1e42407ad8004c1d59c05f791011f195d