Forbid block descriptors in initial xlat table levels

Forbid block descriptors in initial xlat table levels

In AArch64, depending on the granularity of the translation tables,
level 0 and/or level 1 of the translation tables may not support block
descriptors, only table descriptors.

This patch introduces a check to make sure that, even if theoretically
it could be possible to create a block descriptor to map a big memory
region, a new subtable will be created to describe its mapping.

Change-Id: Ieb9c302206bfa33fbaf0cdc6a5a82516d32ae2a7
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>