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
Acked-by: Andreas Bießmann