riscv: sifive: Implement a model for SiFive FU540 OTP

riscv: sifive: Implement a model for SiFive FU540 OTP

This implements a simple model for SiFive FU540 OTP (One-Time
Programmable) Memory interface, primarily for reading out the
stored serial number from the first 1 KiB of the 16 KiB OTP
memory reserved by SiFive for internal use.

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>