drivers/console: Reimplement MUTLI_CONSOLE_API framework in C

Now that we have switched to using the stack in MULTI_CONSOLE_API
framework functions and have factored all code involved in crash
reporting out into a separate file, there's really no reason to keep the
main framework code in assembly anymore. This patch rewrites it in C
which allows us to have a single implementation across aarch32/64 and
should be much easier to maintain going forward.

Change-Id: I6c85a01e89a79e8b233f3f8bee812f0dbd026221
Signed-off-by: Julius Werner <jwerner@chromium.org>