[ALSA] clean up device types symbols

Modules: ALSA Core,ALSA Minor Numbers

Remove the unused and undefined symbols SNDRV_DEVICE_TYPE_{MIXER,
PCM_PLOOP,PCM_CLOOP}, and introduce a new symbol SNDRV_MINOR_GLOBAL
for non-card-specific devices like the sequencer or the timer.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>