xen/arm: Initial support for PSCI-0.2

xen/arm: Initial support for PSCI-0.2

This patch adds SMC calls to suport a subset of PSCI-0.2 functions
(PSCI_VERSION, CPU_ON, SYSTEM_OFF, SYSTEM_RESET).

By default, the psci_init() will use PSCI-0.2. Otherwise, it will
use PSCI-0.1 if PSCI-0.2 fails or un-supported.

To add support for PSCI_VERSION, this patch replaces the "bool_t psci_available"
variable with "int psci_ver", which contains the PSCI_VERSION as described in the
PSCI-0.2 spec.

Signed-off-by: Suravee Suthikulpanit <Suravee.Suthikulpanit@amd.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>