board/avnet: swap ramdisk and DT load addresses in U-Boot environment

board/avnet: swap ramdisk and DT load addresses in U-Boot environment

As reported in bug #9091, the U-Boot environment we provide for the
Microzed and Zedboard platforms loads the ramdisk at 0x2000000 and the
DT at 0x3000000. This means that a large enough ramdisk overwrites the
DT. It makes more sense to load the DT at 0x2000000 and the ramdisk at
0x3000000.

Reported-by:  Michael Monaghan <michaellmonaghan@gmail.com>
Tested-by: Michael Monaghan <michaellmonaghan@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>