doc: Update memory layout for firmware configuration area

Captured the increase in firmware configuration area from
4KB to 8kB in memory layout document. Updated the documentation
to provide details about fw_config separately.

Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
Change-Id: Ifbec443ced479301be65827b49ff4fe447e9109f