infra/common: update RAS documentation

Update RAS documentation. This patch adds following contents
- Creates a RAS folder to collate all RAS docs
- Adds kernel first and firmware first error handling information
- Adds error injection feature information
- Adds detail explanation and supportive pseudo code to perform error
  injection on N2 CPU and Secure RAM in their respective docs

Signed-off-by: Omkar Anand Kulkarni <omkar.kulkarni@arm.com>
Change-Id: I5f27b650c61d2a45816181674164f01721673755