MFC r261083, r261092, r261126, r261127, r261128, r261129, r261130, r261131

  Add Atmel serial drivers.

  uart clock-frequency is a FreeBSD-specific extention. Make it optional and
  allow the client uart drivers to decide if a frequency is required.

  Remove redundant declaration for uart devclass.
  Commit some unrelated, but harmless, FDT ifdefs.

  Add at91 NAND support, Fix comment.

  Remove obsolete options and fix comments
MFC r258044, r258679, r263990

Adds gpioiic.4 and gpioled.4 man pages.  Moves some of the information that
was previously available on gpio.4 to their respectives pages.  Add the
cross references on gpioctl.8.

Add gpiobus(4) as a link to gpio(4).