mfd: at91-usart: Add platform dependency

It doesn't make sense to present option MFD_AT91_USART by default if
not building an AT91 kernel, as the drivers which depend on it are
not available.

Fixes: 7d3aa342 ("mfd: at91-usart: Add MFD driver for USART")
Signed-off-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/mfd/Kconfig

@@ -102,6 +102,7 @@ config MFD_AAT2870_CORE
 config MFD_AT91_USART
 	tristate "AT91 USART Driver"
 	select MFD_CORE
+	depends on ARCH_AT91 || COMPILE_TEST
 	help
 	  Select this to get support for AT91 USART IP. This is a wrapper
 	  over at91-usart-serial driver and usart-spi-driver. Only one function