omap2: Make OMAP2PLUS select OMAP_DM_TIMER

omap2: Make OMAP2PLUS select OMAP_DM_TIMER

It appears that arch/arm/mach-omap2/timer-gp.o is needed by OMAP2PLUS.
Since timer-gp.o has direct dependencies on omap_dm_timer_*, it seems
correct for the architecture to depend upon OMAP_DM_TIMER, otherwise
unsuspecting integrators will be faced with a whole bunch of linker
errors at the end of their kernel build:

arch/arm/mach-omap2/built-in.o: In function `omap2_gp_timer_set_mode':
/home/bgamari/trees/linux-2.6/arch/arm/mach-omap2/timer-gp.c:84: undefined reference to `omap_dm_timer_stop'
/home/bgamari/trees/linux-2.6/arch/arm/mach-omap2/timer-gp.c:88: undefined reference to `omap_dm_timer_get_fclk'
/home/bgamari/trees/linux-2.6/arch/arm/mach-omap2/timer-gp.c:90: undefined reference to `omap_dm_timer_set_load_start'
...

Signed-off-by: Ben Gamari <bgamari.foss@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>