ARM: dts: STi: Add fake reg property for sti-display-subsystem

As sti-display-subsystem sub-nodes (sti-compositor, sti-tvout
sti-hdmi, sti-hda and sti-hqvdp) are SoC's IP, we add a fake
reg property.
This allows to fix the following warning when compiling
dtb with W=1 option:

arch/arm/boot/dts/stih410-b2120.dtb: Warning (unit_address_vs_reg):
Node /soc/sti-display-subsystem/sti-hda@8d02000 has a unit name, but no reg property

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>