Most devices are using the same default omap_device_pm_latency structure
during device built. In order to avoid the duplication of the same
structure everywhere, add a default structure that will be used if
the device does not have an explicit one.

Next patches will clean the duplicated structures.
Signed-off-by: Benoit Cousson <b-cousson@ti.com>
Cc: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>