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
diff --git a/plat/common/src/plat_common_init.c b/plat/common/src/plat_common_init.c
index bdf50af..20634d2 100644
--- a/plat/common/src/plat_common_init.c
+++ b/plat/common/src/plat_common_init.c
@@ -160,7 +160,7 @@
 
 	ret = xlat_ctx_cfg_init(&runtime_xlat_ctx_cfg, VA_LOW_REGION,
 				&static_regions[0], nregions + COMMON_REGIONS,
-				VIRT_ADDR_SPACE_SIZE);
+				PLAT_CMN_VIRT_ADDR_SPACE_SIZE);
 
 	if (ret != 0) {
 		ERROR("%s (%u): %s (%i)\n",