chore(cpus): optimise runtime errata applications

The errata framework has a helper to invoke workarounds, complete with a
cpu rev_var check. We can use that directly instead of the
apply_cpu_pwr_dwn_errata to save on some code, as well as an extra
branch. It's also more readable.

Also, apply_erratum invocation in cpu files don't need to check the
rev_var as that was already done by the cpu_ops dispatcher for us to end
up in the file.

Finally, X2 erratum 2768515 only applies in the powerdown sequence, i.e.
at runtime. It doesn't achieve anything at reset, so we can label it
accordingly.

Change-Id: I02f9dd7d0619feb54c870938ea186be5e3a6ca7b
Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
diff --git a/include/lib/cpus/aarch64/cpu_macros.S b/include/lib/cpus/aarch64/cpu_macros.S
index 98294b9..5e92934 100644
--- a/include/lib/cpus/aarch64/cpu_macros.S
+++ b/include/lib/cpus/aarch64/cpu_macros.S
@@ -433,7 +433,7 @@
  *
  * _get_rev:
  *	Optional parameter that determines whether to insert a call to the CPU revision fetching
- *	procedure. Stores the result of this in the temporary register x10.
+ *	procedure. Stores the result of this in the temporary register x10 to allow for chaining
  *
  * clobbers: x0-x10 (PCS compliant)
  */