drm/imagination: Add RISC-V firmware processor support

drm/imagination: Add RISC-V firmware processor support

Newer PowerVR GPUs (such as the BXS-4-64 MC1) use a RISC-V firmware
processor instead of the previous MIPS or META.

The current version of this patch depends on a patch[1] which exists in
drm-misc-fixes, but has not yet made it back to drm-misc-next (the
target of this patch). That patch adds the function pvr_vm_unmap_obj()
which is used here.

[1]: https://lore.kernel.org/r/20250226-hold-drm_gem_gpuva-lock-for-unmap-v2-1-3fdacded227f@imgtec.com

Signed-off-by: Sarah Walker <sarah.walker@imgtec.com>
Reviewed-by: Frank Binns <frank.binns@imgtec.com>
Link: https://lore.kernel.org/r/20250410-sets-bxs-4-64-patch-v1-v6-14-eda620c5865f@imgtec.com
Signed-off-by: Matt Coster <matt.coster@imgtec.com>