ACPICA: FADT support cleanup

ACPICA commit 34ccd43af3fd1870fddfac0617dd0ba706963558

Remove all vestiges of the version 2 FADT which never was included
in the ACPI specification.

This enabled significant cleanup of both the data table compiler
and the disassembler.

Added many clarification comments to associate each FADT version
with the version of the ACPI spec where it was originally
defined.

Link: https://github.com/acpica/acpica/commit/34ccd43aSigned-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/acpica/tbfadt.c

@@ -476,17 +476,19 @@ static void acpi_tb_convert_fadt(void)
 	u32 i;
 
 	/*
-	 * For ACPI 1.0 FADTs (revision 1 or 2), ensure that reserved fields which
+	 * For ACPI 1.0 FADTs (revision 1), ensure that reserved fields which
 	 * should be zero are indeed zero. This will workaround BIOSs that
 	 * inadvertently place values in these fields.
 	 *
 	 * The ACPI 1.0 reserved fields that will be zeroed are the bytes located
 	 * at offset 45, 55, 95, and the word located at offset 109, 110.
 	 *
-	 * Note: The FADT revision value is unreliable. Only the length can be
-	 * trusted.
+	 * Note: The FADT revision value is unreliable because of BIOS errors.
+	 * The table length is instead used as the final word on the version.
+	 *
+	 * Note: FADT revision 3 is the ACPI 2.0 version of the FADT.
 	 */
-	if (acpi_gbl_FADT.header.length <= ACPI_FADT_V2_SIZE) {
+	if (acpi_gbl_FADT.header.length <= ACPI_FADT_V3_SIZE) {
 		acpi_gbl_FADT.preferred_profile = 0;
 		acpi_gbl_FADT.pstate_control = 0;
 		acpi_gbl_FADT.cst_control = 0;