Update Linux to v5.4.2

Change-Id: Idf6911045d9d382da2cfe01b1edff026404ac8fd
diff --git a/Documentation/firmware-guide/index.rst b/Documentation/firmware-guide/index.rst
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+.. SPDX-License-Identifier: GPL-2.0
+
+===============================
+The Linux kernel firmware guide
+===============================
+
+This section describes the ACPI subsystem in Linux from firmware perspective.
+
+.. toctree::
+   :maxdepth: 1
+
+   acpi/index
+