The board select menu in arch/arm/Kconfig is still big.
To slim down it, this commit moves AT91 boards to
arch/arm/mach-at91/Kconfig.
Also, consolidate "config SYS_SOC" in each board Kconfig.

The Kconfig files under board/ directory were modified with the
following command:

    find board -name Kconfig | xargs sed -i -e '
    /config SYS_SOC/ {
        N
        /default "at91"/ {
            N
            d
        }
    }
    '

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Andreas Bießmann <andreas.devel@googlemail.co>