ALSA: core: add API header and driver header files

This patch adds the header files for ioctl definitions and header file for
driver APIs for lower level device drivers to use

Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>