xlat lib v2: Refactor the functions enabling the MMU

xlat lib v2: Refactor the functions enabling the MMU

This patch refactors both the AArch32 and AArch64 versions of the
function enable_mmu_arch().

In both versions, the code now computes the VMSA-related system
registers upfront then program them in one go (rather than interleaving
the 2).

In the AArch64 version, this allows to reduce the amount of code
generated by the C preprocessor and limits it to the actual differences
between EL1 and EL3.

In the AArch32 version, this patch also removes the function
enable_mmu_internal_secure() and moves its code directly inside
enable_mmu_arch(), as it was its only caller.

Change-Id: I35c09b6db4404916cbb2e2fd3fda2ad59f935954
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>