MAINTAINERS: powerpc: Add Maddy

Maddy will be helping out with upstream maintenance, add him as a
reviewer.

Acked-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240827063651.28985-1-mpe@ellerman.id.au