Add enable mmu platform porting interfaces

Previously, the enable_mmu_elX() functions were implicitly part of
the platform porting layer since they were included by generic
code. These functions have been placed behind 2 new platform
functions, bl31_plat_enable_mmu() and bl32_plat_enable_mmu().
These are weakly defined so that they can be optionally overridden
by platform ports.

Also, the enable_mmu_elX() functions have been moved to
lib/aarch64/xlat_tables.c for optional re-use by platform ports.
These functions are tightly coupled with the translation table
initialization code.

Fixes ARM-software/tf-issues#152

Change-Id: I0a2251ce76acfa3c27541f832a9efaa49135cc1c
diff --git a/plat/fvp/platform.mk b/plat/fvp/platform.mk
index 105cdfa..4cc4d1e 100644
--- a/plat/fvp/platform.mk
+++ b/plat/fvp/platform.mk
@@ -55,6 +55,7 @@
 				lib/aarch64/xlat_tables.c			\
 				lib/semihosting/semihosting.c			\
 				lib/semihosting/aarch64/semihosting_call.S	\
+				plat/common/aarch64/plat_common.c		\
 				plat/fvp/fvp_io_storage.c
 
 BL1_SOURCES		+=	drivers/arm/cci400/cci400.c			\