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.