[M] Change in coreboot[master]: soc/intel: Use common codeflow for MP init

[Arthur Heymans (Code Review)]

Attention is currently required from: Nico Huber, Sean Rhodes, Tarun Tuli, Subrata Banik, Jonathan Zhang, Johnny Lin, Kapil Porwal, Christian Walter, Lean Sheng Tan, Werner Zeh, Tim Chu.

Arthur Heymans has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/72604 )

Change subject: soc/intel: Use common codeflow for MP init
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Patch Set 2:

(1 comment)

File src/soc/intel/apollolake/cpu.c:

https://review.coreboot.org/c/coreboot/+/72604/comment/8a467063_fab87e3d 
PS2, Line 267: 	if (CONFIG(SOC_INTEL_COMMON_BLOCK_CPU_MPINIT))
             : 		return;
> This seems to be here because APL and GLK use different paths.
> `cpu_bus_ops->init` used to bail out here on GLK (because the
> common code handled it). But now common code would indirectly
> call this function...
>
> I think we are in trouble here, because of the singleton global
> `mp_init_cpus` symbol. (soc/ code seems to be full of such single-
> tons... not sure if that's a good path to follow)
>
> If I understand things correctly, one way out of it would be
> to avoid the `.init` for GLK, or have a function there that
> calls mp_cpus_bus_init() conditionally. (Or figure out why APL
> still doesn't use common code, ofc.)

I think APL did not need MP init to happen before calling FSP-S