add Alphascale to vendor-prefixes.txt

this company already provided some products, so it make sense to add
them to vendor-prefixes.txt list

Signed-off-by: Oleksij Rempel <linux@rempel-privat.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Olof Johansson <olof@lixom.net>