libnvdimm, pfn, dax: show supported dax/pfn region alignments in sysfs

The alignment of a DAX and PFN regions dictates the page sizes that can
be used to map the region. Even if the hardware page sizes are known the
actual range of supported page sizes that can be used with DAX depends
on the kernel configuration. As a result it's best that the kernel
advertises the alignments that should be used with these region types.

This patch adds the 'supported_alignments' region attribute to expose
this information to userspace.

Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
[djbw: integrate with nd_size_select_show() rename and other fixups]
Signed-off-by: Dan Williams <dan.j.williams@intel.com>