ARM64 / ACPI: Introduce early_param "acpi=" to enable/disable ACPI

This implements the following policy to decide whether ACPI should
be used to boot the system:
- acpi=off: ACPI will not be used to boot the system, even if there is
  no alternative available (e.g., device tree is empty)
- acpi=force: only ACPI will be used to boot the system; if that fails,
  there will be no fallback to alternative methods (such as device tree)
- otherwise, ACPI will be used as a fallback if the device tree turns out
  to lack a platform description; the heuristic to decide this is whether
  /chosen is the only node present at depth 1

CC: Catalin Marinas <[email protected]>
CC: Will Deacon <[email protected]>
CC: Rafael J. Wysocki <[email protected]>
Acked-by: Olof Johansson <[email protected]>
Acked-by: Grant Likely <[email protected]>
Tested-by: Timur Tabi <[email protected]>
Signed-off-by: Al Stone <[email protected]>
Signed-off-by: Graeme Gregory <[email protected]>
Signed-off-by: Hanjun Guo <[email protected]>
Signed-off-by: Ard Biesheuvel <[email protected]>
Signed-off-by: Will Deacon <[email protected]>

Documentation/kernel-parameters.txt

@@ -165,7 +165,7 @@ multipliers 'Kilo', 'Mega', and 'Giga', equalling 2^10, 2^20, and 2^30
 bytes respectively. Such letter suffixes can also be entirely omitted.
 
 
-	acpi=		[HW,ACPI,X86]
+	acpi=		[HW,ACPI,X86,ARM64]
 			Advanced Configuration and Power Interface
 			Format: { force | off | strict | noirq | rsdt }
 			force -- enable ACPI if default was off
@@ -175,6 +175,7 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 				strictly ACPI specification compliant.
 			rsdt -- prefer RSDT over (default) XSDT
 			copy_dsdt -- copy DSDT to memory
+			For ARM64, ONLY "acpi=off" or "acpi=force" are available
 
 			See also Documentation/power/runtime_pm.txt, pci=noacpi