hw/ssi: Document ssi_transfer() method

A SPI transaction consists of shifting bit in sync with the CLK
line, writing on the MOSI (output) line / and reading MISO (input)
line.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Gustavo Romero <gustavo.romero@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
1 file changed