MicroSemi Switchtec management interface driver

Microsemi's "Switchtec" line of PCI switch devices is already well
supported by the kernel with standard PCI switch drivers.  However, the
Switchtec device advertises a special management endpoint with a separate
PCI function address and class code.  This endpoint enables some additional
functionality which includes:

 * Packet and Byte Counters
 * Switch Firmware Upgrades
 * Event and Error logs
 * Querying port link status
 * Custom user firmware commands

Add a switchtec kernel module which provides PCI driver that exposes a char
device.  The char device provides userspace access to this interface
through read, write and (optionally) poll calls.

A userspace tool and library which utilizes this interface is available
at [1].  This tool takes inspiration (and borrows some code) from
nvme-cli [2].  The tool is largely complete at this time but additional
features may be added in the future.

[1] https://github.com/sbates130272/switchtec-user
[2] https://github.com/linux-nvme/nvme-cli



[Dan Carpenter <dan.carpenter@oracle.com>: don't invert error codes]
[Christophe JAILLET <christophe.jaillet@wanadoo.fr>: fix
switchtec_dev_open() error handling]
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>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>