ASoC: Add missed MODULE_LICENSE("GPL") for imx-pcm-fiq

This driver can be built as module and the file header indicates that
the driver is published under the GPL.
Thus add MODULE_LICENSE("GPL") for it.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>