MAINTAINERS: Add memory_mapping.h and memory_mapping.c to "Memory API"

MAINTAINERS: Add memory_mapping.h and memory_mapping.c to "Memory API"

Both files logically belong to "Memory API" and are not yet listed
anywhere else explicitly. Let's add them to "Memory API".

Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>
Message-Id: <20210723100532.27353-4-david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>