ASoC: fsl: SND_SOC_FSL_AUD2HTX should depend on ARCH_MXC

The Freescale/NXP AUDIO TO HDMI TX module is only present on NXP i.MX 8
Series SoCs.  Hence add a dependency on ARCH_MXC, to prevent asking the
user about this driver when configuring a kernel without i.MX 8 platform
support.

Fixes: 8a24c834 ("ASoC: fsl_aud2htx: Add aud2htx module driver")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20201110145120.3280658-1-geert+renesas@glider.be


Signed-off-by: Mark Brown <broonie@kernel.org>