Becuase the board select menu in arch/arm/Kconfig is too big,
move the Zynq board select menu to zynq/Kconfig.

Consolidate also common settings (CONFIG_SYS_CPU="armv7" and
CONFIG_SYS_SOC="zynq").

Refactor board/xilinx/zynq/MAINTAINERS too.
Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Tested-by: Michal Simek <michal.simek@xilinx.com>