alsa-info: provide DMI system sku

alsa-info: provide DMI system sku

The system sku is used in SOF machine drivers to set specific quirks,
expose it in alsa-info to help support users. Example on a SoundWire
enabled device:

cat /sys/class/dmi/id/product_sku
0A32
dmidecode -s system-sku-number
0A32

Add a variable and expose the results of both methods.

Note that the dmidecode support is very recent and might not yet be
available in all distros:

http://git.savannah.nongnu.org/cgit/dmidecode.git/commit/?id=d70d5e686148eabe90c89fbf4cdcf5258db5aa05

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>