ARM: dts: stm32: keep uart nodes behavior on stm32mp15xx-dhcor-avenger96

DMA configuration is added to uart nodes in stm32mp15x device tree.
Delete uart4 DMA property in stm32mp15xx-dhcor-avenger96 board device
tree to keep console in irq mode, as DMA support for console has been
removed from the driver by commit e359b441 ("serial: stm32: fix
threaded interrupt handling").
Delete also usart2 and uart7 DMA property 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>