i.mx: fsl_esdhc: add the i.mx6q support

i.mx: fsl_esdhc: add the i.mx6q support

The mmc host controller on the i.mx6q is called usdhc which
is redesigned based on the freescale esdhc controller.

The usdhc controller is almost compatible with esdhc except
it adds one mix register to support debug/SD3.0 and move
the low bit 0-6 of XFERTYP register to the mix control reg
low bit 0-6. Thus on i.mx6q, we have the following compared
with the previous soc: (can refer to RM of chapter 56.3.3)

i.mx6q:
mix control:
bit 31 - bit 7: Added for debug/SD3.0 support
bit 6  - bit 0: move in the XFERTYP register bit 6-0 on previous soc
XFERTYP register:
bit 31 - bit 7: the same as before,
bit 6  - bit 0: no-use

previous soc
mix control: no
XFERTYP register:
bit 31 - bit 0: xfertype information

Signed-off-by: Jason Liu <jason.hui@linaro.org>
Cc: Andy Fleming <afleming@gmail.com>
Cc: Stefano Babic <sbabic@denx.de>
Acked-by: Stefano Babic <sbabic@denx.de>