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/units/fifo/nvgpu-fifo.c

@@ -145,6 +145,8 @@ int test_fifo_remove_support(struct unit_module *m,
 		g->fifo.remove_support(&g->fifo);
 	}
 
+	test_fifo_cleanup_gv11b_reg_space(m, g);
+
 	return UNIT_SUCCESS;
 }