keystone: kconfig: move board select menu and common settings

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

Move also common settings (CONFIG_SYS_CPU="armv7" and
CONFIG_SYS_SOC="keystone").

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Reviewed-by: Tom Rini <trini@ti.com>
Cc: Vitaly Andrianov <vitalya@ti.com>