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/wb45n_defconfig b/configs/wb45n_defconfig
index d770e00..ef069a6 100644
--- a/configs/wb45n_defconfig
+++ b/configs/wb45n_defconfig
@@ -40,6 +40,7 @@
 CONFIG_NAND_ATMEL=y
 CONFIG_PMECC_CAP=4
 CONFIG_SPL_GENERATE_ATMEL_PMECC_HEADER=y
+CONFIG_ATMEL_USART=y
 CONFIG_LZMA=y
 CONFIG_LZO=y
 CONFIG_OF_LIBFDT=y