arm64: dts: lx2160a: Correct CPU core idle state name

lx2160a support PW15 but not PW20, correct name to avoid confusing.

Signed-off-by: Ran Wang <ran.wang_1@nxp.com>
Fixes: 00c5ce8a ("arm64: dts: lx2160a: add cpu idle support")
Acked-by: Li Yang <leoyang.li@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>