arm64: dts: mt8192: Change idle states names to reflect actual function

arm64: dts: mt8192: Change idle states names to reflect actual function

The names of the idle states are misleading being this a single cluster
SoC, a cluster-sleep idle state is impossible!

After some research in ATF, it emerged that the cpu-sleep state is in
reality putting CPUs in retention state, while the cluster-sleep one
is turning off the CPUs.

Summarizing renaming:
 - cpu-sleep -> cpu-retention
 - cluster-sleep -> cpu-off

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20230126103526.417039-7-angelogioacchino.delregno@collabora.com
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>