ACPI / debugger: Add AML debugger documentation

This patch documents the AML debugger.

Signed-off-by: Lv Zheng <lv.zheng@intel.com>
[ rjw: Editorial changes ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

diff --git a/Documentation/acpi/aml-debugger.txt b/Documentation/acpi/aml-debugger.txt
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+The AML Debugger
+
+Copyright (C) 2016, Intel Corporation
+Author: Lv Zheng <lv.zheng@intel.com>
+
+
+This document describes the usage of the AML debugger embedded in the Linux
+kernel.
+
+1. Build the debugger
+
+   The following kernel configuration items are required to enable the AML
+   debugger interface from the Linux kernel:
+
+   CONFIG_ACPI_DEBUGGER=y
+   CONFIG_ACPI_DEBUGGER_USER=m
+
+   The userspace utlities can be built from the kernel source tree using
+   the following commands:
+
+   $ cd tools
+   $ make acpi
+
+   The resultant userspace tool binary is then located at:
+
+     tools/acpi/power/acpi/acpidbg/acpidbg
+
+   It can be installed to system directories by running "make install" (as a
+   sufficiently privileged user).
+
+2. Start the userspace debugger interface
+
+   After booting the kernel with the debugger built-in, the debugger can be
+   started by using the following commands:
+
+   # mount -t debugfs none /sys/kernel/debug
+   # modprobe acpi_dbg
+   # tools/acpi/power/acpi/acpidbg/acpidbg
+
+   That spawns the interactive AML debugger environment where you can execute
+   debugger commands.
+
+   The commands are documented in the "ACPICA Overview and Programmer Reference"
+   that can be downloaded from
+
+   https://acpica.org/documentation
+
+   The detailed debugger commands reference is located in Chapter 12 "ACPICA
+   Debugger Reference".  The "help" command can be used for a quick reference.
+
+3. Stop the userspace debugger interface
+
+   The interactive debugger interface can be closed by pressing Ctrl+C or using
+   the "quit" or "exit" commands.  When finished, unload the module with:
+
+   # rmmod acpi_dbg
+
+   The module unloading may fail if there is an acpidbg instance running.
+
+4. Run the debugger in a script
+
+   It may be useful to run the AML debugger in a test script. "acpidbg" supports
+   this in a special "batch" mode.  For example, the following command outputs
+   the entire ACPI namespace:
+
+   # acpidbg -b "namespace"