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 <catalin.marinas@arm.com>
CC: Will Deacon <will.deacon@arm.com>
CC: Rafael J. Wysocki <rjw@rjwysocki.net>
Acked-by: NOlof Johansson <olof@lixom.net>
Acked-by: NGrant Likely <grant.likely@linaro.org>
Tested-by: NTimur Tabi <timur@codeaurora.org>
Signed-off-by: NAl Stone <al.stone@linaro.org>
Signed-off-by: NGraeme Gregory <graeme.gregory@linaro.org>
Signed-off-by: NHanjun Guo <hanjun.guo@linaro.org>
Signed-off-by: NArd Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: NWill Deacon <will.deacon@arm.com>