ARM: dts: STi: Add fake reg for restart, powerdown and picophy/softreset

Add fake reg property for restart, powerdown, picophyreset
and softreset.
This allows to fix the following warning when compiling
dtb with W=1 option:

arch/arm/boot/dts/stih407-b2120.dtb: Warning (simple_bus_reg):
Node /soc/restart missing or empty reg/ranges property
arch/arm/boot/dts/stih407-b2120.dtb: Warning (simple_bus_reg):
Node /soc/powerdown-controller missing or empty reg/ranges property
arch/arm/boot/dts/stih407-b2120.dtb: Warning (simple_bus_reg):
Node /soc/softreset-controller missing or empty reg/ranges property
arch/arm/boot/dts/stih407-b2120.dtb: Warning (simple_bus_reg):
Node /soc/picophyreset-controller missing or empty reg/ranges property

arch/arm/boot/dts/stih410-b2120.dtb: Warning (simple_bus_reg):
Node /soc/restart missing or empty reg/ranges property
arch/arm/boot/dts/stih410-b2120.dtb: Warning (simple_bus_reg):
Node /soc/powerdown-controller missing or empty reg/ranges property
arch/arm/boot/dts/stih410-b2120.dtb: Warning (simple_bus_reg):
Node /soc/softreset-controller missing or empty reg/ranges property
arch/arm/boot/dts/stih410-b2120.dtb: Warning (simple_bus_reg):
Node /soc/picophyreset-controller missing or empty reg/ranges property

arch/arm/boot/dts/stih410-b2260.dtb: Warning (simple_bus_reg):
Node /soc/restart missing or empty reg/ranges property
arch/arm/boot/dts/stih410-b2260.dtb: Warning (simple_bus_reg):
Node /soc/powerdown-controller missing or empty reg/ranges property
arch/arm/boot/dts/stih410-b2260.dtb: Warning (simple_bus_reg):
Node /soc/softreset-controller missing or empty reg/ranges property
arch/arm/boot/dts/stih410-b2260.dtb: Warning (simple_bus_reg):
Node /soc/picophyreset-controller missing or empty reg/ranges property

arch/arm/boot/dts/stih418-b2199.dtb: Warning (simple_bus_reg):
Node /soc/restart missing or empty reg/ranges property
arch/arm/boot/dts/stih418-b2199.dtb: Warning (simple_bus_reg):
Node /soc/powerdown-controller missing or empty reg/ranges property
arch/arm/boot/dts/stih418-b2199.dtb: Warning (simple_bus_reg):
Node /soc/softreset-controller missing or empty reg/ranges property
arch/arm/boot/dts/stih418-b2199.dtb: Warning (simple_bus_reg):
Node /soc/picophyreset-controller missing or empty reg/ranges property

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