amd64_edac: build driver only on AMD hardware

-tip testing found the following build failure (config attached):

drivers/built-in.o: In function `amd64_check':
amd64_edac.c:(.text+0x3e9491): undefined reference to `amd_decode_nb_mce'
drivers/built-in.o: In function `amd64_init_2nd_stage':
amd64_edac.c:(.text+0x3e9b46): undefined reference to `amd_report_gart_errors'
amd64_edac.c:(.text+0x3e9b55): undefined reference to `amd_register_ecc_decoder'
drivers/built-in.o: In function `amd64_nbea_store':
amd64_edac_dbg.c:(.text+0x3ea22e): undefined reference to `amd_decode_nb_mce'
drivers/built-in.o: In function `amd64_remove_one_instance':
amd64_edac.c:(.devexit.text+0x3eea): undefined reference to `amd_report_gart_errors'
amd64_edac.c:(.devexit.text+0x3ef6): undefined reference to `amd_unregister_ecc_decoder'

the AMD EDAC code has a dependency on CONFIG_CPU_SUP_AMD facilities. The
patch below solves the problem here.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>