ARM: OMAP: omap_device: Add omap_device_[alloc|delete] for DT integration

Split the omap_device_build_ss into two smaller functions
that will allow to populate a platform_device already allocated by
device-tree.

The functionality of the omap_device_build_ss is still the same, but
the omap_device_alloc will be usable with devices already built by
device-tree.

Signed-off-by: Benoit Cousson <b-cousson@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>