omap2+: Use dmtimer macros for clockevent

This patch makes timer-gp.c to use only a subset of dmtimer
functions without the need to initialize dmtimer code early.

Also note that now with the inline functions, timer_set_next_event
becomes more efficient in the lines of assembly code.

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