ARM: imx6: Add DHCOM i.MX6 PDK board support

Add support for the DHCOM i.MX6 PDK board. This board has:
- FEC ethernet
- EHCI USB host
- 3x SDMMC

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>