ARM: imx: don't build ssi-fiq if not required

The symbols provided by ssi-fiq are used in sound/soc/fsl/imx-pcm-fiq.c
only. Build ssi-fiq.o/ssi-fiq-ksym.o only if SND_SOC_IMX_PCM_FIQ is
enabled.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>