riscv: sifive_u: Instantiate OTP memory with a serial number

riscv: sifive_u: Instantiate OTP memory with a serial number

This adds an OTP memory with a given serial number to the sifive_u
machine. With such support, the upstream U-Boot for sifive_fu540
boots out of the box on the sifive_u machine.

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>