docs: document the errata framework

Also add a recommended Procedure Call Standard (PCS) to use inside CPU
files and split everything into sections to make it easier to follow.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: Idd876d8e598b5dfe1193aa3e7375c52f6edf5671
(cherry picked from commit 6a0e8e80fbc8665f000fd3619ca5315b120ead83)
2 files changed