SPM: Refactor xlat context creation

SPM: Refactor xlat context creation

Right now the virtual address space is fixed to
PLAT_VIRT_ADDR_SPACE_SIZE, so all base translation tables are the same
size and need the same alignment. The current code allocates the exact
space needed by this initial table.

However, a following patch is going to allow each partition to choose
the size of its address space based on the memory regions defined in
their resource description, so it isn't possible to determine this at
build time. As this optimization no longer applies, it has to be
removed.

Change-Id: Ia8d19f4981e1017e4ffe0ba136de73d701044cb0
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>