ARM: at91: move board select menu and common settings

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>
diff --git a/configs/at91sam9x5ek_dataflash_defconfig b/configs/at91sam9x5ek_dataflash_defconfig
index d00eed5..15b5fa9 100644
--- a/configs/at91sam9x5ek_dataflash_defconfig
+++ b/configs/at91sam9x5ek_dataflash_defconfig
@@ -1,3 +1,4 @@
 CONFIG_SYS_EXTRA_OPTIONS="AT91SAM9X5,SYS_USE_DATAFLASH"
 CONFIG_ARM=y
+CONFIG_ARCH_AT91=y
 CONFIG_TARGET_AT91SAM9X5EK=y