Factor out translation table setup in ARM FVP port

This patch factors out the ARM FVP specific code to create MMU
translation tables so that it is possible for a boot loader stage to
create a different set of tables instead of using the default ones.
The default translation tables are created with the assumption that
the calling boot loader stage executes out of secure SRAM. This might
not be true for the BL3_2 stage in the future.

A boot loader stage can define the `fill_xlation_tables()` function as
per its requirements. It returns a reference to the level 1
translation table which is used by the common platform code to setup
the TTBR_EL3.

This patch is a temporary solution before a larger rework of
translation table creation logic is introduced.

Change-Id: I09a075d5da16822ee32a411a9dbe284718fb4ff6
diff --git a/bl2/bl2.ld.S b/bl2/bl2.ld.S
index 77eb201..d30149b 100644
--- a/bl2/bl2.ld.S
+++ b/bl2/bl2.ld.S
@@ -61,7 +61,7 @@
     } >RAM
 
     /*
-     * The .xlat_table section is for full, aligned page tables (4K).
+     * The xlat_table section is for full, aligned page tables (4K).
      * Removing them from .bss avoids forcing 4K alignment on
      * the .bss section and eliminates the unecessary zero init
      */
@@ -93,6 +93,15 @@
     } >RAM
 
     /*
+     * The .xlat_table section is for full, aligned page tables (4K).
+     * Removing them from .bss avoids forcing 4K alignment on
+     * the .bss section and eliminates the unecessary zero init
+     */
+    xlat_table (NOLOAD) : {
+        *(xlat_table)
+    } >RAM
+
+    /*
      * The base address of the coherent memory section must be page-aligned (4K)
      * to guarantee that the coherent data are stored on their own pages and
      * are not mixed with normal data.  This is required to set up the correct