Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Merge r207585 from cas(4):
- Don't probe for PHYs if we already know to use a SERDES. Unlike as with
cas(4) this only serves to speed up the the device attach though and can
only be determined via the OFW device tree but not from the VPD.
- Don't touch the MIF when using a SERDES.
- Add some missing bus space barriers, mainly in the PCS code path.

- Initialize the ifnet structure, especially if_dname, before probing
the PHYs as some PHY drivers use it (but probably shouldn't). How
gem(4) has worked with brgphy(4) on powerpc without this so far is
unclear to me.
- Introduce a dying flag which is set during detach and checked in
gem_ioctl() in order to prevent active BPF listeners to clear
promiscuous mode which may lead to the tick callout being restarted
which will trigger a panic once it's actually gone.
- In gem_stop() reset rather than just disable the transmitter and
receiver in order to ensure we're not unloading DMA maps still in
use by the hardware. [1]
- The blanking time is specified in PCI clocks so we should use twice
the value when operating at 66MHz.
- Spell some 2 as ETHER_ALIGN and a 19 as GEM_STATUS_TX_COMPLETION_SHFT
to make the actual intentions clear.
- As we don't unload the peak attempts counter ignore its overflow
interrupts.
- Remove a stale setting of a variable to GEM_TD_INTERRUPT_ME which
isn't used afterwards.
- For optimum performance increment the TX kick register in multiples
of 4 if possible as suggested by the documentation.
- Partially revert r164931; drivers should only clear the watchdog
timer if all outstanding TX descriptors are done.
- Fix some debugging strings.
- Add a missing BUS_DMASYNC_POSTWRITE in gem_rint().
- As the error paths in the interrupt handler are generally unlikely
predict them as false.
- Add support for the SBus version of the GEM controller. [2]
- Add some lock assertions.
- Improve some comments.
- Fix some more or less cosmetic issues in the code of the PCI front-end.
- Change some softc members to be unsigned where more appropriate and
remove unused ones.

Approved by: re (kib)
Obtained from: NetBSD (partially) [2], OpenBSD [1]
MFC after: 2 weeks

Build a gem module, for sparc64 only for now.