ASoC: imx-ssi: remove unneeded mach/hardware.h inclusion

The inclusion of mach/hardware.h is not used by the driver at all.
Remove it.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Cc: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: alsa-devel@alsa-project.org