arm/arm64: psci: Don't assume method is hvc

The method can be smc in addition to hvc, and it will be when running
on bare metal. Additionally, we move the invocations to assembly so
we don't have to rely on compiler assumptions. We also fix the
prototype of psci_invoke. function_id should be an unsigned int, not
an unsigned long.

Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>
Tested-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>