gpu: nvgpu: mig: Defer dev_nodes creation and create new power node to support MIG

- This is deferring the dev_nodes creation after power_on to
select the MIG config and to create the dev_nodes as per the
selected MIG config.

- The patch is adding a device node to issue power on. The
nodes are:
 for igpu :/dev/nvgpu/igpu0/power
 for dgpu:/dev/nvgpu/dgpu-0001:01:00.0/power

To issue power on :
echo "1" > /dev/nvgpu/igpu0/power
echo "1" > /dev/nvgpu/dgpu-0001:01:00.0/power

JIRA NVGPU-6633

Change-Id: Ic4f1f3e42724cc788dcfaf0e881d188fd3bd1ce1
Signed-off-by: dt <dt@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/c/linux-nvgpu/+/2512647
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>

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

@@ -0,0 +1,29 @@
+/*
+ * Copyright (c) 2021, 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 NVGPU_POWER_LINUX_H
+#define NVGPU_POWER_LINUX_H
+
+#include <nvgpu/types.h>
+
+int gk20a_power_open(struct inode *inode, struct file *filp);
+ssize_t gk20a_power_read(struct file *filp, char __user *buf,
+		                size_t size, loff_t *off);
+ssize_t gk20a_power_write(struct file *filp, const char __user *buf,
+		                size_t size, loff_t *off);
+int gk20a_power_release(struct inode *inode, struct file *filp);
+
+#endif