soc/intel/skylake: Move MAX_CPUS from mb to SoC

Configure MAX_CPUS in SoC Kconfig with 8 as default value and remove it
from every mainboard where 8 is used.

Change-Id: I825625bf842e8cd22dada9a508a7176e5cc2ea57
Signed-off-by: Felix Singer <felixsinger@posteo.net>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/49105
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Reviewed-by: Michael Niewöhner <foss@mniewoehner.de>
Reviewed-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/intel/skylake/Kconfig

@@ -80,6 +80,10 @@ config CPU_SPECIFIC_OPTIONS
 	select UDELAY_TSC
 	select UDK_2015_BINDING
 
+config MAX_CPUS
+	int
+	default 8
+
 config FSP_HYPERTHREADING
 	bool "Enable Hyper-Threading"
 	default y