ARM: dts: stm32: preset stm32mp15x video #address- and #size-cells

The cell count for address and size is defined by the binding and not
something a board would change. Avoid each board adding this
boilerplate by having the cell size specification in the SoC DTSI.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>