arm/acpi: Parse FADT table and get PSCI flags

arm/acpi: Parse FADT table and get PSCI flags

There are two flags: PSCI_COMPLIANT and PSCI_USE_HVC. When set, the
former signals to the OS that the hardware is PSCI compliant. The latter
selects the appropriate conduit for PSCI calls by toggling between
Hypervisor Calls (HVC) and Secure Monitor Calls (SMC). FADT table
contains such information, parse FADT to get the flags for furture
usage.

Since STAO table and the GIC version are introduced by ACPI 6.0, we will
check the version and only parse FADT table with version >= 6.0. If
firmware provides ACPI tables with ACPI version less than 6.0, OS will
be messed up with those information, so disable ACPI if we get an FADT
table with version less than 6.0.

Signed-off-by: Hanjun Guo <hanjun.guo@linaro.org>
Signed-off-by: Naresh Bhat <naresh.bhat@linaro.org>
Signed-off-by: Parth Dixit <parth.dixit@linaro.org>
Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>