feat(sgi): deviate from arm css common uart related definitions

The Neoverse reference design platforms will migrate to use different
set of secure and non-secure UART ports. This implies that the board
specific macros defined in the common Arm platform code will no longer
be usable for Neoverse reference design platforms.

In preparation for migrating to a different set of UART ports, add a
Neoverse reference design platform specific copy of the board
definitions. The value of these definitions will be changed in
subsequent patches.

Signed-off-by: Rohit Mathew <rohit.mathew@arm.com>
Change-Id: I39170848ecd81a7c1bbd3689bd905e45f9435f5c