MFC r271054:

 When built with FDT support, add /boot/dtb to the list of search directories.
MFC r271055, r271084, r271094:

  Add a function to get the frequency of the AHB bus.  Another stopgap
  function until we have full clock support for imx6.

  The imx5x and imx6 chips have an onboard IOMUX device which also contains a
  few "general purpose registers" whose values control chip behavior in ways
  that have nothing to do with IO pin mux control.  Define a simple API that
  other soc-specific code can use to read and write the registers, and provide
  the imx51 implementation of them.

  Fix a typo.
MFC r271057: Create a /boot/dtb directory to house DTB blobs.
MFC r268834, r268835:

 o Enable GPIO device driver for i.MX6.
   It was originally written for i.MX5 and compatible with newer chip.
 o Extend device tree information
 o style(9) fixes
 o Rename gpio driver file.
MFC r268973, r268977: Rename i.MX I2C driver file, enable it on imx6.