ARM: AM43xx: hwmod: set DSS submodule parent hwmods

Set DSS core hwmod as the parent for all the DSS submodules. This fixes
the boot time DSS reset, removing the following warnings:

omap_hwmod: dss_dispc: cannot be enabled for reset (3)
omap_hwmod: dss_rfbi: cannot be enabled for reset (3)

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