gpu: nvgpu: Debugfs for Volt

Changes:
1. Reading operating voltage and minimum voltage of chip.
2. Added nvgpu_volt_init_debugfs for creating the debugfs interface.
3. Command for reading current voltage:
   cat /sys/kernel/debug/gpu_pci/volt/current_voltage
4. Command for reading minimum voltage:
   cat /sys/kernel/debug/gpu_pci/volt/minimum_voltage

Jira NVGPU-1731

Change-Id: I76d42fc376235f37c44d259398bd6f125a90413b
Signed-off-by: rmylavarapu <rmylavarapu@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/1986403
GVS: Gerrit_Virtual_Submit
Reviewed-by: Vijayakumar Subbu <vsubbu@nvidia.com>
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>

drivers/gpu/nvgpu/os/linux/debug_volt.h

@@ -0,0 +1,29 @@
+/*
+ * Copyright (c) 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,
+ * version 2, as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef __DEBUG_VOLT_H
+#define __DEBUG_VOLT_H
+
+#ifdef CONFIG_DEBUG_FS
+int nvgpu_volt_init_debugfs(struct gk20a *g);
+#else
+static inline int nvgpu_volt_init_debugfs(struct gk20a *g)
+{
+	return 0;
+}
+#endif
+
+#endif