lava: Switch amd64 reference board from Minnowboard to UP Squared 6000

The Apertis amd64 reference platform -MinnowBoard Turbot board- went out
of production a while ago and are no longer available. The UP Squared
6000 board has been chosen as the new reference platform, so let's
replace it.

Signed-off-by: Ariel D'Alessandro <ariel.dalessandro@collabora.com>