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>