Add i.MX7D based Meerkat96 board support

The Meerkat96 board, based on the NXP i.MX7D SoC, is a member of
96Boards community and complies with all Consumer Edition board
specifications.

https://www.novtech.com/products/meerkat96.html
https://www.96boards.org/product/imx7-96/



The initial supported/tested devices include:
 - Debug serial
 - SD
 - USB Host (with Ethernet)

With these support, it's good enough for loading Linux Kernel from SD or
Ethernet over USB.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Tested-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>