plat: imx7: Add PicoPi iMX7D basic support

The PicoPi iMX7D is a 2 board development board consisting of
a System-on-Module and a carrier baseboard and optimized for
the Internet-of-Things (IoT).

This patch add basic support to this board.

Signed-off-by: Jun Nie <jun.nie@linaro.org>
Reviewed-by: Louis Mayencourt <louis.mayencourt@arm.com>
Change-Id: I009d85819c4f73b7063aab73d0f6ee74e6ef3fc4