platform/chrome: pstore: fix dmi table to match all chrome systems

Turns out that DMI_SYSTEM_VENDOR is actually the native vendor of each
Chromebook/box. I tested the original patch on a Pixel that -- surprise,
has Google as vendor. *facepalm*.

The only other data I can think of to probe on is Google_* in the version
string.  Checking with our firmware team, all systems should have this
and nothing else than Chrome hardware should have the coreboot + Google_*
combination to date.

So, we'll switch to this. For future platforms we are going to move to
using an ACPI device to configure this instead of a DMI table (yay!),
so longer-term that will sort itself out.

Signed-off-by: Olof Johansson <olof@lixom.net>
Reviewed-by: Benson Leung <bleung@chromium.org>