ARM: OMAP2+: Move dmtimer clock set function to dmtimer driver

OMAP1 uses an architecture specific function for setting the dmtimer clock
source, where as the OMAP2+ devices use the clock framework. Eventually OMAP1
device should also use the clock framework and hence we should not any
architecture specific functions.

For now move the OMAP2+ function for configuring the clock source into the
dmtimer driver. Therefore, we do no longer need to specify an architecture
specific function for setting the clock source for OMAP2+ devices. This will
simplify device tree migration of the dmtimers for OMAP2+ devices.

From now on, only OMAP1 devices should specify an architecture specific
function for setting the clock source via the platform data set_dmtimer_src()
function pointer.

Signed-off-by: Jon Hunter <jon-hunter@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>