arm64: dts: sprd: Add clock reference for pll2 on UMS512

Fix below dtbs_check warning:

'clocks' is a dependency of 'clock-names'

Link: https://lore.kernel.org/r/20231221092824.1169453-2-chunyan.zhang@unisoc.com


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