MFC r260886, r261165, r261171, r261172, r261214

  Fix gcc with -Wstrict-prototypes by telling it bi_emac takes no
  parameters.

  Bus space handles need to be the VA of the requested resource, not the
  rounded page VA. Correct so the DBGU device can be mapped for FDT
  console since it isn't on a page boundary.

  Make early printf output nicer by inserting a carriage return before
  any linefeeds that are output.

  Before resetting the USART, delay a bit to allow the transmitter to
  finish the current character to drain to avoid glitching. Also,
  simplify the code a smidge.

  Remove extra parens to silence clang warning.
MFC r258209, r258210, r261211,

  Add a driver for the Texas Instruments Mailbox hardware.

  Add a driver for the TI Programmable Realtime Unit Subsystem.

  fix args to mtx_init