Documentation: ACPI for ARM64

Add documentation for the guidelines of how to use ACPI
on ARM64.

Reviewed-by: Suravee Suthikulpanit <Suravee.Suthikulpanit@amd.com>
Reviewed-by: Yi Li <phoenix.liyi@huawei.com>
Reviewed-by: Mark Langsdorf <mlangsdo@redhat.com>
Reviewed-by: Ashwin Chaugule <ashwinc@codeaurora.org>
Acked-by: Robert Richter <rrichter@cavium.com>
Signed-off-by: Graeme Gregory <graeme.gregory@linaro.org>
Signed-off-by: Al Stone <al.stone@linaro.org>
Signed-off-by: Hanjun Guo <hanjun.guo@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>