armv8: move reset branch into boot hook

The boot0 hook we have so far is applied _after_ the initial branch
to the "reset" entry point. An upcoming change requires even this
branch to be changed, so we apply the hook macro at the earliest
point, and have the branch in the hook file as well.
This is no functional change at this point, just refactoring to simplify
upcoming patches.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagan Teki <jagan@openedev.com>