riscv/sifive_u: Add a serial property to the sifive_u SoC

riscv/sifive_u: Add a serial property to the sifive_u SoC

At present the board serial number is hard-coded to 1, and passed
to OTP model during initialization. Firmware (FSBL, U-Boot) uses
the serial number to generate a unique MAC address for the on-chip
ethernet controller. When multiple QEMU 'sifive_u' instances are
created and connected to the same subnet, they all have the same
MAC address hence it creates a unusable network.

A new "serial" property is introduced to the sifive_u SoC to specify
the board serial number. When not given, the default serial number
1 is used.

Suggested-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>