Add support for the USB PHY on imx6 SoCs.  Pretty minimal at this point,
but enough to get usb host mode working.
Add stubbed-out imx6 support for clocks and power management.  This
contains little more than a few stub functions required to keep the
linker happy, but it's enough to let early imx6 development proceed.
Add sdhci driver glue for imx family SoCs.  This should support both uSDHC
(newer SoCs) and eSDHC (older SoCs), but the eSDHC support is untested and
likely to need some tweaking.