ARM: zynq: drop "optional" from board select in favor of ZC702

One disadvantage of commit a26cd049

 (arch: Make board selection
choices optional) is that Kconfig could create such an insane
.config file that no board is selected.

Rip off the "optional" again in favor of ZC702 as the default
target.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>