ARM: OMAP: Add DMTIMER definitions for posted mode

For OMAP2+ devices, when using DMTIMERs for system timers (clock-events and
clock-source) the posted mode configuration of the timers is used. To allow
the compiler to optimise the functions for configuring and reading the system
timers, the posted flag variable is hard-coded with the value 1. To make it
clear that posted mode is being used add some definitions so that it is more
readable.

Signed-off-by: Jon Hunter <jon-hunter@ti.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>