pmem: Initial version of persistent memory driver

PMEM is a new driver that presents a reserved range of memory as a
block device.  This is useful for developing with NV-DIMMs, and
can be used with volatile memory as a development platform.

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
pmem: Add top level README.md explaining PMEM

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>