nvdimm: allow exposing RAM carveouts as NVDIMM DIMM devices

There are use cases, for example virtual machine hosts, that create
"persistent" memory regions using memmap= option on x86 or dummy
pmem-region device tree nodes on DT based systems.

Both these options are inflexible because they create static regions and
the layout of the "persistent" memory cannot be adjusted without reboot
and sometimes they even require firmware update.

Add a ramdax driver that allows creation of DIMM devices on top of
E820_TYPE_PRAM regions and devicetree pmem-region nodes.

The DIMMs support label space management on the "device" and provide a
flexible way to access RAM using fsdax and devdax.

Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Link: https://patch.msgid.link/20251026153841.752061-2-rppt@kernel.org
Signed-off-by: Ira Weiny <ira.weiny@intel.com>

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