Hi Sylvain,
You are absolutely right, the u-boot built with xilinx-v14.4 tag will boot Linux without reserving any memory (ignoring what is defined in the devicetree.dtb). This happens because bootm command verifies and alters devicetree.dtb before booting Linux. In the older versions of u-boot the boot was performed without bootm command and modifying the source code of devicetree only was sufficient for getting reserved memory area. Now, when booting with bootm command we have to make an additional changes to u-boot source code (sorry I forgot to update the user guide).
Please add the following line to u-boot-xlnx/include/configs/zynq_zed.h and build new u-boot:
#define CONFIG_SYS_MEM_TOP_HIDE 0x2000000
Thanks,
Roman.