fix(security): update Cortex-A15 CPU lib files for CVE-2022-23960

fix(security): update Cortex-A15 CPU lib files for CVE-2022-23960

Cortex-A15 does not support FEAT_CSV2 so the existing workaround for
Spectre V2 is sufficient to mitigate against Spectre BHB attacks,
however the code needed to be updated to work with the new build flag.

Also, some code was refactored several years ago and not updated in
the Cortex-A15 library file so this patch fixes that as well.

Signed-off-by: John Powell <john.powell@arm.com>
Change-Id: I768c88a38c561c91019b038ac6c22b291955f18e