Change in coreboot[master]: {cpu/x86, drivers/amd}: Use `get_var_mtrr_count()` to get MTRR count

[build bot (Jenkins) (Code Review)]

Attention is currently required from: Subrata Banik, Angel Pons, Nick Vaccaro, Kane Chen, Werner Zeh.
build bot (Jenkins) has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/63219 )

Change subject: {cpu/x86, drivers/amd}: Use `get_var_mtrr_count()` to get MTRR count
......................................................................

Patch Set 2: Verified+1