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Documented the CPU specific build macros created for AT
speculative workaround.

Updated the description of 'ERRATA_SPECULATIVE_AT' errata
workaround option.
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
Change-Id: Ie46a80d4e8183c1d5c8b153f08742a04d41a2af2