armv7: mmu: Make fine grained page tables work across stages

Among its other restrictions (which are noted in a comment above the
function prototype and stay in place), our makeshift fine-grained page
table support for ARM32 has the undocumented feature that it relies on
a global bookkeeping variable, causing all sorts of fun surprises when
you try to use it from multiple stages during the same boot. This patch
redesigns the bookkeeping to stay completely inline in the (persistent)
TTB which should resolve the issue. (This had not been a problem on any
of our platforms for now... I just noticed this because I was trying to
solve the same issue on ARM64.)

BRANCH=None
BUG=None
TEST=Booted veyron_jerry. Mapped a second fine-grained memory range
from romstage, confirmed that it finds the next free spot and leaves the
bootblock table in place.

Change-Id: I325866828b4ff251142e1131ce78b571edcc9cf9
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: http://review.coreboot.org/12074
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>

src/arch/arm/include/armv7/arch/cache.h

@@ -384,7 +384,8 @@ void tlb_invalidate_all(void);
  * Generalized setup/init functions
  */
 
-/* mmu initialization (set page table address, set permissions, etc) */
+/* MMU initialization (set page table base, permissions, initialize subtable
+ * buffer, etc.). Must only be called ONCE PER BOOT, before any mappings. */
 void mmu_init(void);
 
 enum dcache_policy {