riscv: sifive_u: Do not create hard-coded phandles in DT

riscv: sifive_u: Do not create hard-coded phandles in DT

At present the cpu, plic and ethclk nodes' phandles are hard-coded
to 1/2/3 in DT. If we configure more than 1 cpu for the machine,
all cpu nodes' phandles conflict with each other as they are all 1.
Fix it by removing the hardcode.

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>