Various updates:

• gem(4) has its interrupt marked INTR_MPSAFE now.
• sf(4) is locked now.
• sis(4)'s mutex is no longer marked MTX_RECURSE.
• txp(4) and wb(4) locking both need testing.
• Add an entry for vx(4) and note that its locking needs testing. This driver dosen't need bus_dma it appears though as it uses PIO to bang packets into and out of the card.