davinci: kconfig: move board select menu and common settings

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

Move also common settings (CONFIG_SYS_CPU="arm926ejs" and
CONFIG_SYS_SOC="davinci").

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Reviewed-by: Tom Rini <trini@ti.com>
Cc: Heiko Schocher <hs@denx.de>
Cc: Sandeep Paulraj <s-paulraj@ti.com>