xen/arm: Use PSCI-0.2 for machine_halt/restart by default

xen/arm: Use PSCI-0.2 for machine_halt/restart by default

"machine_halt()" and "machine_restart()" are modified to use PSCI interface
by default if PSCI-0.2 is supported. The "raw_machine_reset()" is also removed
since this is unnecessary.

Also, machine_halt() is modified to add 10ms delay to make sure all auxiliary CPUs
are offlined before calling the shutdown.

For non-PSCI, platform_poweroff() and platform_reset() are used instead.

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