hw/riscv: sifive_u: Add a dummy L2 cache controller device

hw/riscv: sifive_u: Add a dummy L2 cache controller device

It is enough to simply map the SiFive FU540 L2 cache controller
into the MMIO space using create_unimplemented_device(), with an
FDT fragment generated, to make the latest upstream U-Boot happy.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <1595227748-24720-1-git-send-email-bmeng.cn@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>