MAINTAINERS: Collect memory device files in "Memory devices"

MAINTAINERS: Collect memory device files in "Memory devices"

Xiao Guangrong doesn't have enough time to actively review or contribute
to our NVDIMM implementation. Let's dissolve the "NVDIMM" section, moving
relevant ACPI parts to "ACPI/SMBIOS" and moving memory device stuff into a
new "Memory devices" section. Make that new section cover other memory
device stuff as well.

We can now drop the "hw/mem/*" rule from "ACPI/SMBIOS". Note that
hw/acpi/nvdimm.c is already covered by "ACPI/SMBIOS".

The following files in hw/mem don't fall into the TYPE_MEMPORY_DEVICE
category:
* hw/mem/cxl_type3.c is CXL specific and belongs to "Compute Express Link"
* hw/mem/sparse-mem.c is already covered by "Device Fuzzing"
* hw/mem/npcm7xx_mc.c is already covered by "Nuvoton NPCM7xx"

Thanks Xiao for your work on NVDIMM!

Cc: Ben Widawsky <ben.widawsky@intel.com>
Cc: Jonathan Cameron <jonathan.cameron@huawei.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Igor Mammedov <imammedo@redhat.com>
Cc: Ani Sinha <ani@anisinha.ca>
Cc: Xiao Guangrong <xiaoguangrong.eric@gmail.com>
Cc: "Philippe Mathieu-Daudé" <f4bug@amsat.org>
Cc: Richard Henderson <richard.henderson@linaro.org>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Julia Suvorova <jusual@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20220617123151.103033-1-david@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>