Cortex-A55: workarounds for errata 1221012

The workaround is added to the Cortex-A55 cpu specific file. The
workaround is disabled by default and have to be explicitly enabled by
the platform integrator.

Change-Id: I3e6fd10df6444122a8ee7d08058946ff1cc912f8
Signed-off-by: Ambroise Vincent <ambroise.vincent@arm.com>