Resolve signed-unsigned comparison issues

A recent commit 030567e6 added U()/ULL()
macro to TF constants. This has caused some signed-unsigned comparison
warnings / errors in the TF static analysis.

This patch addresses these issues by migrating impacted variables from
signed ints to unsigned ints and vice verse where applicable.

Change-Id: I4b4c739a3fa64aaf13b69ad1702c66ec79247e53
Signed-off-by: David Cunado <david.cunado@arm.com>