powerpc: Drop XILINX MAINTAINERS entry

This has been orphaned for ~7 years, remove it.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Grant Likely <grant.likely@arm.com>
Link: https://lore.kernel.org/r/20200224233146.23734-5-mpe@ellerman.id.au