staging: vcsm-cma: Remove cache manipulation ioctl from ARM64

The cache flushing ioctls are used by the Pi3 HEVC hw-assisted
decoder as it needs finer grained flushing control than dma_ops
allow.
These cache calls are not present for ARM64, therefore disable
them. We are not actively supporting 64bit kernels at present,
and the use case of the HEVC decoder is fairly limited.

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