IB/hfi1: Add debugfs to control expansion ROM write protect

Some kernels now enable CONFIG_IO_STRICT_DEVMEM which prevents multiple
handles to PCI resource0. In order to continue to support expansion ROM
updates while the driver is loaded, the driver must now provide an
interface to control the expansion ROM write protection.

This patch adds an exprom_wp debugfs interface that allows the hfi1_eprom
user tool to disable the expansion ROM write protection by opening the
file and writing a '1'.  The write protection is released when writing a
'0' or automatically re-enabled when the file handle is closed.  The
current implementation will only allow one handle to be opened at a time
across all hfi1 devices.

Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Josh Collier <josh.d.collier@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>