ARM: dts: stm32: keep uart nodes behavior on stm32mp15xx-dhcom-pdk2

DMA configuration is added to uart nodes in stm32mp15x device tree.
Delete usart3 and uart8 DMA property in stm32mp15xx-dhcom-pdk2 board
device tree to keep current behavior.

Signed-off-by: Erwan Le Ray <erwan.leray@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>