arm64: dts: ti: k3-am64: Use phandle to stdout UART node

Using a phandle makes it clear which UART we are choosing without needing
to resolve through an alias first.

Especially useful for boards like the TI J721s2-EVM where the alias is
"serial2" but it actually resolves to the 8th UART instance(main_uart8).

Signed-off-by: Andrew Davis <afd@ti.com>
Link: https://lore.kernel.org/r/20230601184933.358731-2-afd@ti.com


Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>