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/smartweb_defconfig b/configs/smartweb_defconfig
index 4545c70..e7edb01 100644
--- a/configs/smartweb_defconfig
+++ b/configs/smartweb_defconfig
@@ -56,6 +56,7 @@
 # CONFIG_SYS_NAND_USE_FLASH_BBT is not set
 CONFIG_NAND_ATMEL=y
 CONFIG_PHYLIB=y
+CONFIG_ATMEL_USART=y
 CONFIG_USB=y
 CONFIG_DM_USB=y
 CONFIG_USB_GADGET=y