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>

sound/soc/pxa/Kconfig

@@ -34,6 +34,7 @@ config SND_PXA2XX_SOC_I2S
 
 config SND_PXA_SOC_SSP
 	tristate "Soc Audio via PXA2xx/PXA3xx SSP ports"
+	depends on PLAT_PXA
 	select PXA_SSP
 	select SND_PXA2XX_LIB