gpu: nvgpu: Improve accuracy of dGPU clk measurement

- This patch replaces "nvgpu_current_time_ms" with "nvgpu_hr_timestamp_us".
- "nvgpu_hr_timestamp_us" gives timestamp in microseconds and has better
accuracy than "nvgpu_current_time_ms" (which gives timestamp in millisecond)

Bug 200503143

Change-Id: I6a10e8e1b3e8ff842aa23f58bf2ba9344af232a6
Signed-off-by: Vaibhav Kachore <vkachore@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/2125959
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>

drivers/gpu/nvgpu/os/linux/soc.c

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2017-2018, NVIDIA CORPORATION.  All rights reserved.
+ * Copyright (c) 2017-2019, NVIDIA CORPORATION.  All rights reserved.
  *
  * This program is free software; you can redistribute it and/or modify it
  * under the terms and conditions of the GNU General Public License,
@@ -127,3 +127,8 @@ int nvgpu_init_soc_vars(struct gk20a *g)
 #endif
 	return 0;
 }
+
+u64 nvgpu_us_counter(void)
+{
+	return (u64)nvgpu_current_time_us();
+}