arm64: dts: mt8192: Fix idle-states nodes naming scheme

Tweak the name of the idle-states subnodes so that they follow the
binding pattern, getting rid of dtbs_check warnings.

Only the usage of "-" in the name was necessary, but "off" was also
exchanged for "sleep" since that seems to be a more common wording in
other dts files.

Fixes: 9260918d

 ("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/20220617233150.2466344-2-nfraprado@collabora.com


Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>