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 <bmeng.cn@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>