refactor: Move the virtual address size config to platform

This patch moves the VIRT_ADDR_SPACE_WIDTH from lib/slot_buf component
to plat/common as this is only needed in that component.
Also the patch adds suitable documentation for the build option.

Signed-off-by: Soby Mathew <soby.mathew@arm.com>
Change-Id: I77663c27a77eee01a173818e42479082e25e9074
4 files changed