Cortex-A76: workarounds for errata 1257314, 1262606, 1262888, 1275112

The workarounds for errata 1257314, 1262606, 1262888 and 1275112 are
added to the Cortex-A76 cpu specific file. The workarounds are disabled
by default and have to be explicitly enabled by the platform integrator.

Change-Id: I70474927374cb67725f829d159ddde9ac4edc343
Signed-off-by: Soby Mathew <soby.mathew@arm.com>