doc: Update description for AT speculative workaround

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