arm64: dts: ti: k3-am654: Populate power-domain property for UART nodes

Populate power-domain property for UART nodes, this is required for
Linux to enable UART clocks via PM calls. Without this UART instances
not initialized by bootloader (like main_uart1) fails to work in Linux.
Also, drop current-speed property from main_uart1 and main_uart2 nodes
as these UARTs are not initialized before Linux boots up and current
speed is unknown.

Signed-off-by: Vignesh R <vigneshr@ti.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>