soc/intel/apollolake: Add Kconfig switch to enable minimum clock ratio

soc/intel/apollolake: Add Kconfig switch to enable minimum clock ratio

Add a Kconfig switch to be able to set the CPU clock to the lowest
possible ratio. If enabled the CPU will consume as little power as
possible while providing the lowest performance.

This setting can be overruled by the OS if it has an p-state driver
which can adjust the clock to its need.

Change-Id: I4a59586da72d1915749110a36f565fe2aa69e073
Signed-off-by: Werner Zeh <werner.zeh@siemens.com>
Reviewed-on: https://review.coreboot.org/c/29772
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>