ASoC: atmel: properly select dma driver state

It is currently possible to have CONFIG_SND_ATMEL_SOC_SSC=y with either
CONFIG_SND_ATMEL_SOC_PDC=m or CONFIG_SND_ATMEL_SOC_DMA=m. This results in a
driver that compiles but does not link with this kind of error:

sound/built-in.o: In function `atmel_ssc_set_audio':
(.text+0x87d90): undefined reference to `atmel_pcm_pdc_platform_register'
sound/built-in.o: In function `atmel_ssc_put_audio':
(.text+0x8879a): undefined reference to `atmel_pcm_pdc_platform_unregister'

Introduce new config options SND_ATMEL_SOC_SSC_PDC and
SND_ATMEL_SOC_SSC_DMA which should be used by the board drivers and the
correct logic to properly select the SND_ATMEL_SOC_PDC and
SND_ATMEL_SOC_DMA states.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Mark Brown <broonie@kernel.org>