ARM: dts: STi: STiH407: Provide generic (safe) DVFS configuration

You'll notice that the voltage cell is populated with 0's.  Voltage
information is very platform specific, even depends on 'cut' and
'substrate' versions.  Thus it is left blank for a generic (safe)
implementation.  If other nodes/properties are provided by the
bootloader, the ST CPUFreq driver will over-ride these generic
values.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Maxime Coquelin <maxime.coquelin@st.com>