[coreboot] Enabling SpeedStep for all Denverton SoC variants

[Sumo]

Hi,

Today denverton_core_init() is enabling the Intel SpeedStep only if turbo
mode is available. As a result, the SpeedStep is not enable for the C3558
variant (and others).  Any clues of why it was implemented this way?
I think it is safe to remove the "if (get_turbo_state() == TURBO_ENABLED)"
test and always enable the SpeedStep technology.

Thanks,
Sumo