Add a function so that OMAP dmtimers can be requested by device-tree
node. This allows for devices, such as the internal DSP, or drivers,
such as PWM, to reference a specific dmtimer node via the device-tree.

Given that there are several APIs available for requesting dmtimers
(by ID, by capability or by node) consolidate the code for all these
functions into a single helper function that can be used by these
request functions.
Signed-off-by: NJon Hunter <jon-hunter@ti.com>
Acked-by: NTony Lindgren <tony@atomide.com>
Signed-off-by: NBenoit Cousson <benoit.cousson@linaro.org>