This adds the SiFive SPI controller model for the FU540 SoC.
The direct memory-mapped SPI flash mode is unsupported.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210126060007.12904-4-bmeng.cn@gmail.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>