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>