ARM: davinci: enable the clocksource driver for DT mode

Switch all davinci boards supporting device tree to using the new
clocksource driver: remove the previous OF_TIMER_DECLARE() from
mach-davinci and select davinci-timer for ARCH_DAVINCI.

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