ARM: dts: STi: Add fake reg property for usb2_picophyX nodes

Add fake reg property for usb2_picophy nodes.
This allows to fix the following warning when compiling dtb
with W=1 option :

arch/arm/boot/dts/stih410-b2120.dtb: Warning (simple_bus_reg):
Node /soc/phy2 missing or empty reg/ranges property
arch/arm/boot/dts/stih410-b2120.dtb: Warning (simple_bus_reg):
Node /soc/phy3 missing or empty reg/ranges property

arch/arm/boot/dts/stih410-b2120.dtb: Warning (simple_bus_reg):
Node /soc/phy2 missing or empty reg/ranges property
arch/arm/boot/dts/stih410-b2120.dtb: Warning (simple_bus_reg):
Node /soc/phy3 missing or empty reg/ranges property

arch/arm/boot/dts/stih418-b2199.dtb: Warning (simple_bus_reg):
Node /soc/phy2 missing or empty reg/ranges property
arch/arm/boot/dts/stih418-b2199.dtb: Warning (simple_bus_reg):
Node /soc/phy3 missing or empty reg/ranges property

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