timer: sandbox: Use device tree to pass the clock frequency

We should use device tree to pass the clock frequency of the timer
instead of hardcoded in the driver codes.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/dts/sandbox.dts b/arch/sandbox/dts/sandbox.dts
index 720ef93..d2addb4 100644
--- a/arch/sandbox/dts/sandbox.dts
+++ b/arch/sandbox/dts/sandbox.dts
@@ -179,6 +179,7 @@
 
 	timer {
 		compatible = "sandbox,timer";
+		clock-frequency = <1000000>;
 	};
 
 	tpm {