Remove coherent memory from the BL memory maps

This patch extends the build option `USE_COHERENT_MEMORY` to
conditionally remove coherent memory from the memory maps of
all boot loader stages. The patch also adds necessary
documentation for coherent memory removal in firmware-design,
porting and user guides.

Fixes ARM-Software/tf-issues#106

Change-Id: I260e8768c6a5c2efc402f5804a80657d8ce38773
diff --git a/plat/juno/juno_private.h b/plat/juno/juno_private.h
index b7ef448..70439e8 100644
--- a/plat/juno/juno_private.h
+++ b/plat/juno/juno_private.h
@@ -134,15 +134,21 @@
 void configure_mmu_el1(unsigned long total_base,
 		       unsigned long total_size,
 		       unsigned long ro_start,
-		       unsigned long ro_limit,
-		       unsigned long coh_start,
-		       unsigned long coh_limit);
+		       unsigned long ro_limit
+#if USE_COHERENT_MEM
+		       , unsigned long coh_start,
+		       unsigned long coh_limit
+#endif
+		       );
 void configure_mmu_el3(unsigned long total_base,
 		       unsigned long total_size,
 		       unsigned long ro_start,
-		       unsigned long ro_limit,
-		       unsigned long coh_start,
-		       unsigned long coh_limit);
+		       unsigned long ro_limit
+#if USE_COHERENT_MEM
+		       , unsigned long coh_start,
+		       unsigned long coh_limit
+#endif
+		       );
 void plat_report_exception(unsigned long type);
 unsigned long plat_get_ns_image_entrypoint(void);
 unsigned long platform_get_stack(unsigned long mpidr);