Emphasize that TF only supports 4 KB granule size

Emphasize that TF only supports 4 KB granule size

At the moment, various parts of the Trusted Firmware code assume
that the granule size used is 4 KB. For example, the linker scripts
enforce 4 KB alignment restrictions on some sections.

However, the ARMv8-A architecture allows 16 KB and 64 KB granule
sizes as well. Some other parts of the TF code, particularly the
architectural code and definitions, have been implemented with
this in mind and cater for all 3 cases.

This discrepancy creates some confusion as to what is effectively
supported in TF. This patch adds some code comments and clarification
in the documentation to make this limitation clearer.

Change-Id: I1f202369b240d8bed9d43d57ecd2a548c86c8598
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>