bus: ti-sysc: Ignore timer12 on secure omap3

Some early omap3 boards use timer12 for system timer, but for secure
SoCs like on n900 it's not accessible. Likely we will be configuring
unavailable devices for other SoCs too based on runtime SoC detection,
so let's use a switch to start with.

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>