ASoC: atmel-pcm-dma: increase buffer_bytes_max

atmel-pcm-dma is not limited to a buffer size of 64kB like atmel-pcm-pdc.
Increase buffer_bytes_max to 512kB to allow for higher bit rates (i.e. 32bps at
192kHz) to work correctly. By default, keep the prealloc at 64kB.

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