[BMG] GSC FW missing from upstream

There's no GSC FW for BMG in upstream, only for LNL: https://git.kernel.org/pub/scm/linux/kernel/git/firmware/linux-firmware.git/tree/xe

So users see warnings like this in their dmesg, as they cannot install FW for it: xe 0000:96:00.0: [drm] gsccs disabled due to lack of FW