spi: s3c24xx: Remove reference to plat/fiq.h

fiq.h contains only a function declaration and is not used by anyone
else. Move the declaration to the driver header file and remove the
unnecessary platform dependency from the driver.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>