firmware/psci: add support for SYSTEM_RESET2

PSCI v1.1 introduced SYSTEM_RESET2 to allow both architectural resets
where the semantics are described by the PSCI specification itself as
well as vendor-specific resets. Currently only system warm reset
semantics is defined as part of architectural resets by the specification.

This patch implements support for SYSTEM_RESET2 by making using of
reboot_mode passed by the reboot infrastructure in the kernel.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Aaro Koskinen <aaro.koskinen@nokia.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>