arm: socfpga: Add entries to enable make dtbs socfpga

Signed-off-by: Dinh Nguyen <dinguyen@altera.com>
Tested-by: Pavel Machek <pavel@denx.de>
Reviewed-by: Pavel Machek <pavel@denx.de>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Olof Johansson <olof@lixom.net>
Cc: Pavel Machek <pavel@denx.de>
Signed-off-by: Olof Johansson <olof@lixom.net>