staging: vchiq_arm: Clean up 40-bit DMA support

Manage the split between addresses for the VPU and addresses for the
40-bit DMA controller with a dedicated DMA device pointer that on non-
BCM2711 platforms is the same as the main VCHIQ device. This allows
the VCHIQ node to stay in the usual place in the DT, and removes the
ugly VC_SAFE macros.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>