arm: kirkwood: make it CONFIG_TIMER aware

If we switch to CONFIG_TIMER, we don't need the legacy timer macros and
functions anymore. Add the proper guards to exclude them from compiling.

Cc: Pali Rohár <pali@kernel.org>
Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>