[ALSA] mpc8610: Add mmap support

Enable mmap support in the MPC8610 ASoC driver.  The driver can use ALSA's
default mmap functionality, it was just not enabled previously.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>