x86: tsc: Rename X86_TSC_TIMER_EARLY_FREQ to X86_TSC_TIMER_FREQ

Currently there are two places to specify the x86 TSC timer frequency
with one in Kconfig used for early timer and the other one in device
tree used when the frequency cannot be determined from hardware.

This may potentially create an inconsistent config where the 2 values
do not match. Let's use the one specified in Kconfig in the device
tree as well.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>