arm64: defconfig: Increase CMA size for VC4

The VC4 needs more memory than the default setting (16 MB):

  vc4-drm soc:gpu: swiotlb: coherent allocation failed, size=16777216
  [drm:vc4_bo_create [vc4]] *ERROR* Failed to allocate from CMA:
  vc4_v3d 3fc00000.v3d: Failed to allocate memory for tile binning: -12.
    You may need to enable CMA or give it more memory.
  vc4-drm soc:gpu: failed to bind 3fc00000.v3d (ops vc4_v3d_ops [vc4]): -12
  vc4-drm soc:gpu: master bind failed: -12
  vc4-drm: probe of soc:gpu failed with error -12

So increase the value to 32 MB and fix this issue.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>