ARM: dts: stm32: keep uart4 and uart7 behavior on stm32mp15xx-dkx

DMA configuration is added to uart nodes in stm32mp15x device tree.
Delete uart4 DMA property in stm32mp15xx-dkx 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 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>