MAINTAINERS: powerpc: Update my status

Maddy is taking over the day-to-day maintenance of powerpc. I will still
be around to help, and as a backup.

Re-order the main POWERPC list to put Maddy first to reflect that.

KVM/powerpc patches will be handled by Maddy via the powerpc tree with
review from Nick, so replace myself with Maddy there.

Remove myself from BPF, leaving Hari & Christophe as maintainers.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>