Convert CONFIG_ATMEL_USART to Kconfig

This converts the following to Kconfig:
   CONFIG_ATMEL_USART

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/at91rm9200ek_defconfig b/configs/at91rm9200ek_defconfig
index 2aaf879..a990360 100644
--- a/configs/at91rm9200ek_defconfig
+++ b/configs/at91rm9200ek_defconfig
@@ -29,6 +29,7 @@
 CONFIG_SYS_FLASH_PROTECTION=y
 CONFIG_SYS_FLASH_CFI=y
 CONFIG_MII=y
+CONFIG_ATMEL_USART=y
 CONFIG_USB=y
 CONFIG_USB_STORAGE=y
 CONFIG_USB_KEYBOARD=y