OMAPDSS: create custom pdevs for DSS omap_devices

Instead of using omap_device_build() to create the omap_devices for DSS
hwmods, create them with a custom function. This will allow us to create
a parent-child hierarchy for the devices so that the omapdss_core device
is parent for the rest of the dss hwmod devices.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>