riscv: Support booting SiFive Unmatched from SPI.

Configure SPI flash devices into SPL.  Add SPI boot option to spl.c.
Document how to format flash for booting.

Signed-off-by: Thomas Skibo <thomas-git@skibo.net>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>
4 files changed