kirkwood: kconfig: refactor Kconfig and defconfig

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

Consolidate also common settings (CONFIG_SYS_CPU="arm926ejs" and
CONFIG_SYS_SOC="kirkwood").

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Prafulla Wadasdkar <prafulla@marvell.com>
Cc: Luka Perkov <luka@openwrt.org>
diff --git a/configs/mv88f6281gtw_ge_defconfig b/configs/mv88f6281gtw_ge_defconfig
index 4e26879..0650032 100644
--- a/configs/mv88f6281gtw_ge_defconfig
+++ b/configs/mv88f6281gtw_ge_defconfig
@@ -1,2 +1,3 @@
 CONFIG_ARM=y
+CONFIG_KIRKWOOD=y
 CONFIG_TARGET_MV88F6281GTW_GE=y