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/slimbootloader.dts b/arch/x86/dts/slimbootloader.dts
index d04095c..9b581c8 100644
--- a/arch/x86/dts/slimbootloader.dts
+++ b/arch/x86/dts/slimbootloader.dts
@@ -7,7 +7,7 @@
 
 /include/ "skeleton.dtsi"
 /include/ "reset.dtsi"
-/include/ "tsc_timer.dtsi"
+#include "tsc_timer.dtsi"
 
 / {
 	model = "slimbootloader x86 payload";