siemens/mc_apl4: Set CPU clock to minimum ratio

The power budget for this mainboard is very limited while the
performance demand is low. Set the CPU clock to the lowest value to
enable maximum efficiency and thus lowest power dissipation.

Change-Id: I23c7c5393deb676b94f2b0ac25e21a7a44cd8cb3
Signed-off-by: Werner Zeh <werner.zeh@siemens.com>
Reviewed-on: https://review.coreboot.org/c/29773
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

src/mainboard/siemens/mc_apl1/variants/mc_apl4/Kconfig

@@ -4,5 +4,6 @@ if BOARD_SIEMENS_MC_APL4
 config BOARD_SPECIFIC_OPTIONS
 	def_bool y
 	select DRIVER_INTEL_I210
+	select APL_SET_MIN_CLOCK_RATIO
 
 endif # BOARD_SIEMENS_MC_APL4