feat(el3-runtime): handle traps for IMPDEF registers accesses

This patch introduces support to handle traps from lower ELs for
IMPDEF system register accesses. The actual support is left to the
platforms to implement.

Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>
Change-Id: I623d5c432b4ce4328b68f238c15b1c83df97c1e5
6 files changed