[M] Change in coreboot[master]: soc: Remove useless extern prototypes in .h files

[Felix Held (Code Review)]

Attention is currently required from: Arthur Heymans, Christian Walter, Elyes Haouas, Fred Reitberger, Hung-Te Lin, Jakub Czapiga, Jason Glenesk, Jeff Daly, Johnny Lin, Julius Werner, Kapil Porwal, Matt DeVillier, Raul Rangel, Subrata Banik, Tarun Tuli, Tim Chu, Vanessa Eusebio, Yidi Lin, Yu-Ping Wu.

Felix Held has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/76859?usp=email )

Change subject: soc: Remove useless extern prototypes in .h files
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Patch Set 1: Code-Review-1

(1 comment)

File src/soc/amd/common/block/include/amdblocks/acpimmio.h:

PS1: 
this will break the build. the actual pointers are defined in soc/amd/common/block/acpimmio/mmio_util.c so the header file that gets included in other places needs to use the extern keyword in order to not redefine the same symbol