ARM: davinci: dm646x: switch to using the clocksource driver

We now have a proper clocksource driver for davinci. Switch the dm646x
platform to using it.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: David Lechner <david@lechnology.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>