x86: dts: Define a default TSC timer frequency

If for some reason, TSC timer frequency cannot be determined from
hardware, nor is it specified in the device tree, U-Boot will panic
resulting in endless reset during boot.

Let's define a default TSC timer frequency using the Kconfig value
CONFIG_X86_TSC_TIMER_FREQ (note: #include must be used instead of
/include/ otherwise the macro is not pre-processed).

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/x86/dts/tsc_timer.dtsi b/arch/x86/dts/tsc_timer.dtsi
index 4f5021d..4df8e9d 100644
--- a/arch/x86/dts/tsc_timer.dtsi
+++ b/arch/x86/dts/tsc_timer.dtsi
@@ -1,6 +1,7 @@
 / {
 	tsc-timer {
 		compatible = "x86,tsc-timer";
+		clock-frequency = <CONFIG_X86_TSC_TIMER_FREQ>;
 		u-boot,dm-pre-reloc;
 	};
 };