armv8r64-extras/demo: Improve the coding of demo program

Refinement on the coding of the demo program, mainly including:
* Fix comments in HTML by adding exclamation mark
* Unify the comment style in C code
* Change the hardcoded numbers to macros
* Fix compile warnings
* Minor fixes on code alignment, padding data structure, etc

Issue-Id: SCM-5615
Signed-off-by: Robbie Cao <robbie.cao@arm.com>
Change-Id: Id075c60933606a6513b6cbb2e9047f494cb0b280