ARM: OMAP2+: Add omap_init_time_of()

This allows us to move the SoCs to probe system timers one SoC
at at time. As arch/arm/mach-omap2/timer.c will be eventually gone,
let's just add omap_init_time_of() to board-generic.c directly.

Cc: Grygorii Strashko <grygorii.strashko@ti.com>
Cc: Keerthy <j-keerthy@ti.com>
Cc: Lokesh Vutla <lokeshvutla@ti.com>
Cc: Rob Herring <robh@kernel.org>
Cc: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>