riscv: sifive_u: Change UART node name in device tree

riscv: sifive_u: Change UART node name in device tree

OpenSBI for fu540 does DT fix up (see fu540_modify_dt()) by updating
chosen "stdout-path" to point to "/soc/serial@...", and U-Boot will
use this information to locate the serial node and probe its driver.
However currently we generate the UART node name as "/soc/uart@...",
causing U-Boot fail to find the serial node in DT.

Signed-off-by: Bin Meng <[email protected]>
Reviewed-by: Alistair Francis <[email protected]>
Signed-off-by: Palmer Dabbelt <[email protected]>