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>

arch/arm/mach-at91/Kconfig

 menuconfig ARCH_AT91
 	bool "Atmel SoCs"
+	depends on ARCH_MULTI_V4T || ARCH_MULTI_V5 || ARCH_MULTI_V7
 	select ARCH_REQUIRE_GPIOLIB
 	select COMMON_CLK_AT91
 	select PINCTRL