Docs: Explain 'Default access rules' feature usage

Feature 'Default access rules' can be used to set up a specific
standalone region. This usage is not explicitly explained in the
document, and here comes the explanation.

Signed-off-by: Ken Liu <Ken.Liu@arm.com>
Change-Id: If394a7bae9e56a76ba7ac810b8809e0ec18f3fde
1 file changed