ideapad-laptop: Add another DMI entry for Yoga 900

This particular laptop has its motherboard replaced and after that, even
with the latest BIOS, some DMI identification strings have become
"INVALID". This includes DMI_PRODUCT_VERSION which results Wifi being
blocked.

It seems that DMI_BOARD_NAME is still valid so use that as an
alternative for Lenovo Yoga 900.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>