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

All Broadwell boards use 8 for MAX_CPUS, so this option can be factored
out into SoC Kconfig.

Change-Id: I311b95ea75a7c6b76b32c7197a0cec86db644234
Signed-off-by: Felix Singer <felixsinger@posteo.net>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/49122
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Michael Niewöhner <foss@mniewoehner.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/intel/broadwell/Kconfig

@@ -39,6 +39,10 @@ config CPU_SPECIFIC_OPTIONS
 	select HAVE_POWER_STATE_AFTER_FAILURE
 	select HAVE_POWER_STATE_PREVIOUS_AFTER_FAILURE
 
+config MAX_CPUS
+	int
+	default 8
+
 config PCIEXP_ASPM
 	bool
 	default y