firmware: dmi: Add DMI_PRODUCT_FAMILY identification string

Sometimes it is more convenient to be able to match a whole family of
products, like in case of bunch of Chromebooks based on Intel_Strago to
apply a driver quirk instead of quirking each machine one-by-one.

This adds support for DMI_PRODUCT_FAMILY identification string and also
exports it to the userspace through sysfs attribute just like the
existing ones.

Suggested-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>