gpu: nvgpu: unit: add SWUTS docs for nvgpu-fifo

This adds the SWUTS documentation for the nvgpu-fifo unit tests:
- test_fifo_init_support
- test_fifo_remove_support

Also added missing register space in remove support.

JIRA NVGPU-3943

Change-Id: I1f3a61cd41af0e8b78b0636adfb3448645e0fc15
Signed-off-by: Thomas Fleury <tfleury@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/2195374
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>

userspace/SWUTS.h

@@ -31,6 +31,7 @@
  * to test the NVGPU driver.
  *
  *   - @ref SWUTS-enabled
+ *   - @ref SWUTS-fifo
  *   - @ref SWUTS-init
  *   - @ref SWUTS-interface-atomic
  *   - @ref SWUTS-mm-allocators-bitmap-allocator