x86: use hardcoded tsc frequency from board configuration

U-Boot crashes when probing SPI flash, or SATA disk.
The call chain is:

<some driver codes> =>
lib/timer.c::udelay() =>
arch/x86/lib/tsc_timer.c::__udelay() =>
arch/x86/lib/tsc_timer.c::get_tbclk_mhz() =>
arch/x86/include/asm/msr.h::native_read_msr(PLATFORM_INFO_MSR)

The crash happens when the code is trying to read an MSR called
PLATFORM_INFO_MSR which is only available on Nehalem/SandyBridge/
IvyBridge/Haswell as of today.

The ideal solution would be to calculate the TSC frequency manually
but for now it's enough to just use a hardcoded value from board
configuration.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>