Revert r269814: blkfront: add support for unmapped IO

Current busdma code for unmapped bios will not properly align the segment
size, causing corruption on blkfront devices. Revert the commit until
busdma code is fixed.

Reported by:	mav
MFC after:	1 day
Implement the same public interface in imx51 and imx6 iomux; use the common
header file for both.  Remove some unused code from imx51_iomux.  The iomux
drivers are required, not optional, adjust the files.* entries accordingly.
The iomux driver is no longer optional, all imx platforms have it as
standard now, so remove it from kernel configs.
Implement the imx_iomux_get/set_gpr() interface for imx6.