Change in coreboot[master]: southbridge/amd: Unify smbus.{c,h} and make them common

[Paul Menzel (Code Review)]

Attention is currently required from: Michał Żygowski, Michał Kopeć, Elyes Haouas.
Paul Menzel has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/61890 )

Change subject: southbridge/amd: Unify smbus.{c,h} and make them common
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Patch Set 4:

(1 comment)

File src/southbridge/amd/common/smbus.h:

https://review.coreboot.org/c/coreboot/+/61890/comment/8079353f_021a2288 
PS4, Line 27: #define AX_INDXC		0
> Strange, as `src/southbridge/amd/cimx/sb800/smbus.h` also defines macros using this define: […]

Some archaeology: Commit 63e62b03a8 (This code provides southbridge initialization for SB800 south bridges. It is dependent on the AMD CIMx/SB800 code.) adds the macros, but they are commented out.

/*//SB00.H
    #define AX_INDXC  0
    #define AX_INDXP  2
    #define AXCFG     4
    #define ABCFG     6
    #define RC_INDXC  1
    #define RC_INDXP  3
    */

Then commit 0141f0d6b8 (sb/amd/*/*/smbus.h: Make 'smbus.h' uniform) remove the commented out section, but misses to comment, that the macros are actually referenced further down.