switchtec: Add user interface documentation

Add standard documentation for the sysfs switchtec attributes and a RST
formatted text file which documents the char device interface.  Jonathan
Corbet has indicated he will move this to a new user-space developer
documentation book once it's created.

Tested-by: Krishna Dhulipala <krishnad@fb.com>
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Stephen Bates <stephen.bates@microsemi.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Wei Zhang <wzhang@fb.com>
Reviewed-by: Jens Axboe <axboe@fb.com>