gpu: nvgpu: add reference to the MC unit in gk20a.h

Add MC unit page reference to the gk20a.h list of units to get
the doxygen contents placed.

JIRA NVGPU-2524

Change-Id: I6bb6178bcb3fea8747582f8416cb9e12fdd9e1c2
Signed-off-by: Sagar Kamble <skamble@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/2226020
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>

drivers/gpu/nvgpu/include/nvgpu/gk20a.h

@@ -52,6 +52,7 @@
  *   - @ref unit-common-nvgpu
  *   - @ref unit-common-ltc
  *   - @ref unit-common-utils
+ *   - @ref unit-mc
  *   - Etc, etc.
  *
  * NVGPU Software Unit Design Documentation