ARM: Fix arm720t SPL build

Take a few SPL fixes from armv7 and apply them to arm720t:
-Use dummy exception handlers for SPL build
-Initialize relocation register r9 to 0 for the case of no relocation
-ifdef out interrupt handler code

Signed-off-by: Allen Martin <amartin@nvidia.com>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Tested-by: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Tom Warren <twarren@nvidia.com>