Specify signed-ness of constants

Specify signed-ness of constants

We relyed on the default signed-ness of constants, which is usually
signed. This can create MISRA violations, such as:

     bl1/bl1_main.c:257:[MISRA C-2012 10.8 (required)] Cast of composite
     expression off essential type signed to essential type unsigned

These constants were only used as unsigned, so this patch makes them
explicitly unsigned.

Change-Id: I5f1310c881e936077035fbb1d5ffb449b45de3ad
Signed-off-by: Jimmy Brisson <jimmy.brisson@arm.com>