ASoC: pxa: make SND_PXA_SOC_SSP depend on PLAT_PXA

For the moment, we can't enable CONFIG_SND_PXA_SOC_SSP unless we are
building for ARM PXA or MMP:

WARNING: unmet direct dependencies detected for PXA_SSP
  Depends on [n]: PLAT_PXA [=n]
  Selected by [y]:
  - SND_PXA_SOC_SSP [=y] && SOUND [=y] && !UML && SND [=y] && SND_SOC [=y]

This adds an explicit dependency for it.

Fixes: 0a94cf34 ("ASoC: pxa: make SND_PXA2XX_SOC_I2S selectable")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mark Brown <broonie@kernel.org>