[ALSA] Update MAINTAINERS for ALSA SoC

Add myself as a point of contact for the ALSA SoC subsystem and add a
reference to the development GIT tree.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lg@opensource.wolfsonmicro.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>