EDAC, ghes: Make platform-based whitelisting x86-only

ARM machines all have DMI tables so if they request hw error reporting
through GHES, then the driver should be able to detect DIMMs and report
errors successfully (famous last words :)).

Make the platform-based list x86-specific so that ghes_edac can load on
ARM.

Reported-by: Qiang Zheng <zhengqiang10@huawei.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: James Morse <james.morse@arm.com>
Tested-by: James Morse <james.morse@arm.com>
Tested-by: Qiang Zheng <zhengqiang10@huawei.com>
Link: https://lkml.kernel.org/r/1526039543-180996-1-git-send-email-zhengqiang10@huawei.com