ARM: dts: stm32: remove now redundant STM32MP15x video cell sizes

With the cell sizes specified in the SoC DTSIs in a previous commit,
individual boards no longer need to specify them, thus drop them.

No functional change.

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