spi: atmel: add support for changing message transfer speed

The only speed available was max_speed (the maximum speed declared for a
device).
This patch adds the support for spi_tranfer->speed_hz parameter.
We can now set a different speed for each spi message.

Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Mark Brown <broonie@linaro.org>