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 <[email protected]>: don't invert error codes]
[Christophe JAILLET <[email protected]>: fix
switchtec_dev_open() error handling]
Tested-by: Krishna Dhulipala <[email protected]>
Signed-off-by: Logan Gunthorpe <[email protected]>
Signed-off-by: Stephen Bates <[email protected]>
Signed-off-by: Bjorn Helgaas <[email protected]>
Reviewed-by: Wei Zhang <[email protected]>
Reviewed-by: Jens Axboe <[email protected]>
Reviewed-by: Greg Kroah-Hartman <[email protected]>

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