xen/arm: move errata CSV2 check earlier

xen/arm: move errata CSV2 check earlier

CSV2 availability check is done after printing to the user that
workaround 1 will be used. Move the check before to prevent saying to the
user that workaround 1 is used when it is not because it is not needed.
This will also allow to reuse install_bp_hardening_vec function for
other use cases.

Code previously returning "true", now returns "0" to conform to
enable_smccc_arch_workaround_1 returning an int and surrounding code
doing a "return 0" if workaround is not needed.

This is part of XSA-398 / CVE-2022-23960.

Signed-off-by: Bertrand Marquis <bertrand.marquis@arm.com>
Reviewed-by: Julien Grall <julien@xen.org>