ARM: at91: add a Kconfig dependency on multi-platform

When building a legacy (non-multi) platforms and if the ARCH_AT91 config option
is enabled there is a build error. We need AT91 to depend on multi-platform
core type options.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Olof Johansson <olof@lixom.net>