ASoC: Make SND_SOC_LITTLEMILL select MFD_WM8994

ASoC: Make SND_SOC_LITTLEMILL select MFD_WM8994

SND_SOC_LITTLEMILL selects SND_SOC_WM8994, but SND_SOC_WM8994 needs MFD_WM8994.
Thus we need to select MFD_WM8994 to fix below build error:

  LD      .tmp_vmlinux1
sound/built-in.o: In function `wm8994_write':
sound/soc/codecs/wm8994.c:201: undefined reference to `wm8994_reg_write'
sound/built-in.o: In function `wm8994_read':
sound/soc/codecs/wm8994.c:222: undefined reference to `wm8994_reg_read'
sound/built-in.o: In function `wm8994_resume':
sound/soc/codecs/wm8994.c:2847: undefined reference to `wm8994_reg_read'
sound/built-in.o: In function `wm8994_codec_probe':
sound/soc/codecs/wm8994.c:3501: undefined reference to `wm8994_reg_read'
sound/soc/codecs/wm8994.c:3660: undefined reference to `wm8994_reg_read'
sound/soc/codecs/wm8994.c:3672: undefined reference to `wm8994_reg_read'
sound/built-in.o: In function `wm8958_dsp2_fw':
sound/soc/codecs/wm8958-dsp2.c:154: undefined reference to `wm8994_bulk_write'
make: *** [.tmp_vmlinux1] Error 1

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>