MAINTAINERS: Add an entry for generic architecture topology

arm and arm64 shared lot of CPU topology related code. This was
consolidated under driver/base/arch_topology.c by Juri. Now RISC-V
is also started sharing the same code pulling more code from arm64
into arch_topology.c

Since I was involved in the review from the beginning, I would like
to assume maintenance for the same.

Cc: Will Deacon <will.deacon@arm.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Juri Lelli <juri.lelli@redhat.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>