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

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

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 specify the board serial
number. When not given, the default serial number 1 is used.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Palmer Dabbelt <palmerdabbelt@google.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <1573916930-19068-1-git-send-email-bmeng.cn@gmail.com>
[ Changed by AF:
 - Use the SoC's serial property to pass the info to the SoC
 - Fixup commit title
 - Rebase on file restructuring
]
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>