ARM: at91/Kconfig: select board files automatically

An explicit selection option is not needed for board files so now we select the
board from SoC option.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
[nicolas.ferre@atmel.com: remove option's comments; split patch]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>