OMAP4: pm.c extensions for OMAP4 support

OMAP4 has an iva device and a dsp devcice where as OMAP2/3
has only an iva device. In this file the iva device in the
system is registered under the name dsp_dev and the API
to retrieve the iva device is omap2_get_dsp_device.
This patch renames the dsp_dev to iva_dev, renames
omap2_get_dsp_device to omap2_get_iva_device,
registers dsp_dev for OMAP4 and adds a new API
omap4_get_dsp_device to retrieve the dep_dev.

Signed-off-by: Thara Gopinath <thara@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>