Correct typographical errors

Corrects typos in core code, documentation files, drivers, Arm
platforms and services.

None of the corrections affect code; changes are limited to comments
and other documentation.

Change-Id: I5c1027b06ef149864f315ccc0ea473e2a16bfd1d
Signed-off-by: Paul Beesley <paul.beesley@arm.com>