microzed: add Avnet Microzed support

Changes are based on Zedboard: Xilinx does not provide a specific DTS
for Microzed in the kernel tree. It is suggested [1] to use the
zynq-zed.dts and change the RAM size

[1] https://forums.xilinx.com/t5/Embedded-Linux/Microzed-default-device-tree-dts/td-p/432856



[Thomas:
 - extend readme.txt to indicate why the same DTB is used for Microzed
   and the Zedboard.
 - add a description + Signed-off-by line in the
   0001-zynq-Create-microzed-specific-U-Boot-environment.patch, by
   re-using the description of the same patch for the Zedboard.]

Signed-off-by: Davide Viti <d.viti@infosolution.it>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>