Move common declarations in reg interface files

Ethos-U65/U85 interface headers have common declarations for some of
the registers. This patch moves the common declarations to a separate
interface header file.

Signed-off-by: Rajasekaran Kalidoss <rajasekaran.kalidoss@arm.com>
Change-Id: I1118d02de9f5195fa333616c4894f8ce91327d3c