[hermon] Perform clean MPT unmap on device shutdown

This change is ported from Flexboot sources.  When stopping a Hermon
device, perform hermon_unmap_mpt() which runs HERMON_HCR_HW2SW_MPT to
bring the Memory Protection Table (MPT) back to software control.

Signed-off-by: Christian Iversen <ci@iversenit.dk>
Modified-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>