MAINTAINERS: Change the x86 microcode loader maintainer

Let's make it official - I've been doing this for a while now anyway.

Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Andreas Herrmann <herrmann.der.user@googlemail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tigran Aivazian <tigran@aivazian.fsnet.co.uk>
Signed-off-by: Ingo Molnar <mingo@kernel.org>