ALSA: pcm - Use pgprot_noncached() for MIPS non-coherent archs

MIPS non-coherent archs need the noncached pgprot in mmap of PCM buffers.
But, since the coherency needs to be checked dynamically via
plat_device_is_coherent(), we need an ugly check dependent on MIPS
in ALSA core code.

This should be cleaned up in MIPS arch side (e.g. creating
dma_mmap_coherent()) in near future.

Tested-by: Wu Zhangjin <wuzhangjin@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>