Add MSI support to puc(9)

Add support for MSI interrupts in the puc(9) driver.  By default the driver
will prefer MSI interrupts to legacy interrupts.  A tunable,
hw.puc.msi_disable, has been added to force the allocation of legacy
interrupts.

Reviewed by:	jhb@
MFC after:	2 weeks
Sponsored by:	Sandvine Inc.
remove link to the missing AMD Geode LX SB man page... we can add it
back once someone cares enough to write one..