configs: rename Zynq defconfig files

configs: rename Zynq defconfig files

These three are Zynq boards and the build procedure is almost the
same.  Having the SoC name prefix "zynq_" would be more consistent.
Also, this is the way in which the Linux Device Trees and the U-Boot
configuration files do.

This commit renames as follows:

   zedboard_defconfig     -> zynq_zed_defconfig
   microzed_defconfig     -> zynq_microzed_defconfig
   xilinx_zc706_defconfig -> zynq_zc706_defconfig

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>