zynq: kconfig: move board select menu and common settings

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>
diff --git a/configs/zynq_zed_defconfig b/configs/zynq_zed_defconfig
index abf7e82..2337906 100644
--- a/configs/zynq_zed_defconfig
+++ b/configs/zynq_zed_defconfig
@@ -1,3 +1,4 @@
 CONFIG_SPL=y
 +S:CONFIG_ARM=y
++S:CONFIG_ZYNQ=y
 +S:CONFIG_TARGET_ZYNQ_ZED=y