Add S390 maintainer information

Add S390 maintainer information

This patch adds information about who handles what when it comes to S390.
I'll gladly support anything that's related to the device emulation model and
S390 KVM parts.

Since this patchset doesn't implement S390 CPU emulation, I left that part
with a question mark. As soon as Uli's patchset gets committed I'd recommend
setting him there.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>