ARM: at91: split out at91x40 into a top-level option

at91x40 is different from all the other at91 machines, and it is
impossible to build a kernel that works on both this SoC and
any of the others, even though it is possible to build a noMMU
kernel for any at91 machine.

By turning at91x40 into a separate top-level option, we explicitly
forbid enabling invalid configurations that include mutually exclusive
machines.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: Andrew Victor <linux@maxim.org.za>
Cc: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>