omap: Set separate timer init functions to avoid cpu_is_omap tests

This is needed for the following patches so we can initialize the
rest of the hardware timers later on.

As with the init_irq calls, there's no need to do cpu_is_omap calls
during the timer init as we only care about the major omap generation.
This means that we can initialize the sys_timer with the .timer
entries alone.

Note that for now we just set stubs for the various sys_timer entries
that will get populated in a later patch. The following patches will
also remove the omap_dm_timer_init calls and change the init for the
rest of the hardware timers to happen with an arch_initcall.

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