infra/common: update SoC expansion block access

Update the documentation about the SoC expansion block connected to the
IO Virtualization block, specifically around the access to SRAM regions
connected to via the IO virtualization block.

Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>
Change-Id: I4b022c208e51a98f368cbcb16841f7421b69057f