ARM: make machine_desc->init_time default to clocksource_of_init

Originally from a default machine descriptor patch from Arnd, pull out
just the default call to clocksource_of_init part. This is needed so that
platforms can simply remove .init_time calls as they are converted to use
clocksource_of_init.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Nicolas Pitre <nico@linaro.org>
Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>