spi: sifive: Add DT documentation for SiFive SPI controller

DT documentation for SPI controller added.

Signed-off-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Emil Renner Berthing <kernel@esmil.dk>
Signed-off-by: Yash Shah <yash.shah@sifive.com>
Signed-off-by: Mark Brown <broonie@kernel.org>