ARM: dts: STi: Add fake reg property for miphy28lp_phy

Add fake reg property to miphy28lp_phy.
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/miphy28lp missing or empty reg/ranges property

arch/arm/boot/dts/stih410-b2120.dtb: Warning (simple_bus_reg):
Node /soc/miphy28lp missing or empty reg/ranges property

arch/arm/boot/dts/stih410-b2260.dtb: Warning (simple_bus_reg):
Node /soc/miphy28lp missing or empty reg/ranges property

arch/arm/boot/dts/stih418-b2199.dtb: Warning (simple_bus_reg):
Node /soc/miphy28lp missing or empty reg/ranges property

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