ARM: DRA7: hwmod: set DSS submodule parent hwmods

Set DSS core hwmod as the parent for all the DSS submodules. This
ensures that the parent hwmods are enabled before any DSS submodules are
accessed.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Paul Walmsley <paul@pwsan.com>