arm64: dts: mt8192: Fix idle-states entry-method

arm64: dts: mt8192: Fix idle-states entry-method

The entry-method property of the idle-states node should be "psci" as
described in the idle-states binding, since this is already the value of
enable-method in the CPU nodes. Fix it to get rid of a dtbs_check
warning.

Fixes: 9260918d3a4f ("arm64: dts: mt8192: Add cpu-idle-states")
Signed-off-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20220617233150.2466344-3-nfraprado@collabora.com
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>