drm/vc4: Add FKMS as an acceptable node for dma ranges.

Under FKMS, the firmware (via FKMS) also requires the VideoCore cache
aliases for image planes, as defined by the dma-ranges under /soc.

Add rpi-firmware-kms to the list of acceptable nodes to look for
to copy dma config from.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>