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/drivers/serial/Makefile b/drivers/serial/Makefile
index 696d5fb..bb99a34 100644
--- a/drivers/serial/Makefile
+++ b/drivers/serial/Makefile
@@ -26,6 +26,7 @@
 LIB	:= $(obj)libserial.a
 
 COBJS-$(CONFIG_ARM_DCC) += arm_dcc.o
+COBJS-$(CONFIG_AT91RM9200_USART) += at91rm9200_usart.o
 COBJS-$(CONFIG_ATMEL_USART) += atmel_usart.o
 COBJS-$(CONFIG_MCFUART) += mcfuart.o
 COBJS-$(CONFIG_NS9750_UART) += ns9750_serial.o