ARM: OMAP2+: Add dmtimer platform function to reserve systimers

ARM: OMAP2+: Add dmtimer platform function to reserve systimers

During early boot, one or two dmtimers are reserved by the kernel as system
timers (for clocksource and clockevents). These timers are marked as reserved
and the dmtimer driver is notified which timers have been reserved via the
platform data information.

For OMAP2+ devices the timers reserved may vary depending on device and compile
flags. Therefore, it is not easy to assume which timers we be reserved for the
system timers. In order to migrate the dmtimer driver to support device-tree we
need a way to pass the timers reserved for system timers to the dmtimer driver.
Using the platform data structure will not work in the same way as it is
currently used because the platform data structure will be stored statically in
the dmtimer itself and the platform data will be selected via the device-tree
match device function (of_match_device).

There are a couple ways to workaround this. One option is to store the system
timers reserved for the kernel in the device-tree and query them on boot.
The downside of this approach is that it adds some delay to parse the DT blob
to search for the system timers. Secondly, for OMAP3 devices we have a
dependency on compile time flags and the device-tree would not be aware of that
kernel compile flags and so we would need to address that.

The second option is to add a function to the dmtimer code to reserved the
system timers during boot and so the dmtimer knows exactly which timers are
being used for system timers. This also allows us to remove the "reserved"
member from the timer platform data. This seemed like the simpler approach and
so was implemented here.

Signed-off-by: Jon Hunter <jon-hunter@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>