at91rm9200: move serial driver to drivers/serial

add CONFIG_AT91RM9200_USART to activate the driver

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
diff --git a/include/configs/m501sk.h b/include/configs/m501sk.h
index a432850..1e7d90e 100644
--- a/include/configs/m501sk.h
+++ b/include/configs/m501sk.h
@@ -110,6 +110,7 @@
 #define CONFIG_CMC_PU2
 
 /* define one of these to choose the DBGU, USART0  or USART1 as console */
+#define CONFIG_AT91RM9200_USART
 #define CONFIG_DBGU
 #undef CONFIG_USART0
 #undef CONFIG_USART1