lrdp-v2: refactor Armv7LPAEMMU code

The LPAE MMU code can be re-used for other modules that need page table
parsing (like the LPAE IOMMU page tables). Refactor the code a bit so
that it can be easily re-used later.

Change-Id: I0deb80a69f2fca83c3ddcc78867fa2e5f5d18bb2