The DEVICE2 memory range is needed to access the Root of Trust Public
Key registers. This is not needed when Trusted Board Boot is disabled
so it's safer to not map it in this case. This also saves one level-2
page table in each of BL1 and BL2 images.

Also add some comments.

Change-Id: I67456b44f3fd5e145f6510a8499b7fdf720a7273
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>