lib/cpus: Report AT speculative erratum workaround

lib/cpus: Report AT speculative erratum workaround

Reported the status (applies, missing) of AT speculative workaround
which is applicable for below CPUs.

 +---------+--------------+
 | Errata  |      CPU     |
 +=========+==============+
 | 1165522 |  Cortex-A76  |
 +---------+--------------+
 | 1319367 |  Cortex-A72  |
 +---------+--------------+
 | 1319537 |  Cortex-A57  |
 +---------+--------------+
 | 1530923 |  Cortex-A55  |
 +---------+--------------+
 | 1530924 |  Cortex-A53  |
 +---------+--------------+

Also, changes are done to enable common macro 'ERRATA_SPECULATIVE_AT'
if AT speculative errata workaround is enabled for any of the above
CPUs using 'ERRATA_*' CPU specific build macro.

Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
Change-Id: I3e6a5316a2564071f3920c3ce9ae9a29adbe435b