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>

arch/arm64/boot/dts/sprd/ums512.dtsi

@@ -291,6 +291,7 @@ anlg_phy_gc_regs: syscon@323e0000 {
 			pll2: clock-controller@0 {
 				compatible = "sprd,ums512-gc-pll";
 				reg = <0x0 0x100>;
+				clocks = <&ext_26m>;
 				clock-names = "ext-26m";
 				#clock-cells = <1>;
 			};