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: 9260918d3a4f ("arm64: dts: mt8192: Add cpu-idle-states")
Signed-off-by: Nícolas F. R. A. Prado <nfraprado@xxxxxxxxxxxxx>