add device nodes for alsa support

target/foxg20/files/etc/mdev.conf

@@ -7,4 +7,10 @@ tty 0:0 666
 ttyS* 0:0 640
 device-mapper 0:0 660 @mkdir /dev/mapper
 tun 0:0 660 >net/tun
+audio root:audio 660 >snd/audio
+dsp root:audio 660 >snd/dsp
+timer root:audio 660 >snd/timer
+controlC0 root:audio 660 >snd/controlC0
+pcmC0D0c root:audio 660 >snd/pcmC0D0c
+pcmC0D0p root:audio 660 >snd/pcmC0D0p
 .* 0:0 644 @/lib/mdev/init