gpu: nvgpu: unit: add --driver-load-path arg

Add --driver-load-path option to specify which library to
load for driver.
There is already an --unit-load-path option that can be
used to specify where to load units from.

Example usage:
./nvgpu_unit --nvtest --unit-load-path units/dgpu \
      --no-color --num-threads 1 \
      --driver-load-path ./libnvgpu-drv-dgpu.so

Jira NVGPU-5217

Change-Id: I6af5d2029138b25a6715154779b812d30052e9e9
Signed-off-by: Thomas Fleury <tfleury@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/c/linux-nvgpu/+/2333498
Reviewed-by: automaticguardword <automaticguardword@nvidia.com>
Reviewed-by: svc-mobile-coverity <svc-mobile-coverity@nvidia.com>
Reviewed-by: svc-mobile-misra <svc-mobile-misra@nvidia.com>
Reviewed-by: Deepak Nibade <dnibade@nvidia.com>
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>
GVS: Gerrit_Virtual_Submit

userspace/include/unit/core.h

@@ -42,7 +42,7 @@ struct unit_fw {
 	struct unit_results	 *results;
 
 	/*
-	 * nvgpu-drv interface. Currently the only two directly referenced
+	 * driver library interface. Currently the only two directly referenced
 	 * functions are:
 	 *
 	 *   nvgpu_posix_probe()