ARC: HSDK: Fix timer frequency value

CPU (and hence cpu timers) on HSDK board runs at 500MHz after
preloader so fix wrong CPU frequency value in hsdk.dts

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>

arch/arc/dts/hsdk.dts

@@ -20,7 +20,7 @@
 		core_clk: core_clk {
 			#clock-cells = <0>;
 			compatible = "fixed-clock";
-			clock-frequency = <1000000000>;
+			clock-frequency = <500000000>;
 			u-boot,dm-pre-reloc;
 		};
 	};