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>