x86/amd_nb: Make amd_northbridges internal to amd_nb.c

Hide amd_northbridges in amd_nb.c so that external callers will have to
use the exported accessor functions.

Also, fix some checkpatch.pl warnings.

Signed-off-by: Yazen Ghannam <Yazen.Ghannam@amd.com>
Cc: linux-edac <linux-edac@vger.kernel.org>
Cc: x86-ml <x86@kernel.org>
Link: http://lkml.kernel.org/r/1478812257-5424-2-git-send-email-Yazen.Ghannam@amd.com
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>