switchtec: Add sysfs attributes to the Switchtec driver

Add a few read-only sysfs attributes which provide some device information
that is exposed from the devices, primarily component and device names and
versions.

These are documented in Documentation/ABI/testing/sysfs-class-switchtec.

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>