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/snapper9g20_defconfig b/configs/snapper9g20_defconfig
index e1c5c2e..abe0333 100644
--- a/configs/snapper9g20_defconfig
+++ b/configs/snapper9g20_defconfig
@@ -36,5 +36,6 @@
 CONFIG_MTD_RAW_NAND=y
 # CONFIG_SYS_NAND_USE_FLASH_BBT is not set
 CONFIG_NAND_ATMEL=y
+CONFIG_ATMEL_USART=y
 CONFIG_USB=y
 CONFIG_USB_STORAGE=y