[bwn] add N-PHY related register defintions.

• Add the siba bus phy/mac/bandwidth clock definitions (TGSLOW*)
• Add the PHY-N register gateway (BWN_PHY_N())
• Add the PHY-N TX phystat1 register - we need to actually fill out more of the PHY encoding information when we assemble a frame.
• Various ancillary stuff

Nothing uses this yet, but I do have CCK/OFDM somewhat working
in 2GHz mode on a PHY-N device.

Obtained from: b43 (definitions)