lib: arm/arm64: Remove unused CPU_OFF parameter

The first version of PSCI required an argument for CPU_OFF, the power_state
argument, which was removed in version 0.2 of the specification [1].
kvm-unit-tests supports PSCI 0.2, and KVM ignores any CPU_OFF parameters,
so let's remove the PSCI_POWER_STATE_TYPE_POWER_DOWN parameter.

[1] ARM DEN 0022D, section 7.3.

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