MAINTAINERS: powerpc: Mark Maddy as "M"

Mark Maddy as a co-maintainer, so that he can get a kernel.org account
and help manage the powerpc tree on kernel.org.

Acked-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://patch.msgid.link/20241115045442.675721-1-mpe@ellerman.id.au