core-for-CI: drm/i915/gsc: define gsc fw

The GSC Firmware for MTL is not quite ready yet, so the FW team has asked us to not push the blob to linux-firmware yet. The expectation is for the FW to be ready around the middle of July.

In theory the patch could be pushed to drm-intel-gt-next - it's a platform both "not normally enabled" since it's under force-probe and "not generally available publicly". However, if the firmware blob is not pushed to the upstream linux-firmware repository, distros may start to see a warning when creating their initrd and the firmware will not be available at the normal place. Also, there isn't much point in going through all the possible bumps through the upstream firmware repo: it will just unnecessarily increase its size.

So commit in core-for-CI is to get this done for MTL now, to start getting CI results for the relevant i915 code.