arm64: dts: sprd: Change UMS512 idle-state nodename to match bindings

Fix below dtbs_check warning:

idle-states: 'core-pd' does not match any of the regexes: '^(cpu|cluster)-', 'pinctrl-[0-9]+'

Link: https://lore.kernel.org/r/20231221092824.1169453-3-chunyan.zhang@unisoc.com


Signed-off-by: Chunyan Zhang <chunyan.zhang@unisoc.com>