infra/common: document RAS architecture and software support

Add a document to explain about general RAS concept. Also touch upon the
software approaches available to achieve RAS error handling on Neoverse
reference design platforms.

Signed-off-by: Omkar Anand Kulkarni <omkar.kulkarni@arm.com>
Change-Id: Ie5e4cb0a4920c6096caf6cfa08dca78a00f911b2