ASoC: samsung: WM8994 depends on MFD_WM8994

Any driver that selects SND_SOC_WM8994 should also make sure that
MFD_WM8994 is set, since the codec relies on the mfd code:

  sound/built-in.o: In function `wm8994_read':
  last.c:(.text+0x20160): undefined reference to `wm8994_reg_read'
  sound/built-in.o: In function `wm8994_write':
  last.c:(.text+0x20e68): undefined reference to `wm8994_reg_write'

This solves the problem by selecting the MFD driver directly
and adding extra 'depends on' statements to make sure that we
respect the dependencies of that driver.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>