omap: Move dmtimer defines to dmtimer.h

These will be needed when dmtimer platform init code gets split
for omap1 and omap2+. These will also be needed for separate
sys_timer init and driver init for the rest of the hardware timers
in the following patches. No functional changes.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Kevin Hilman <khilman@ti.com>