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/wb50n_defconfig b/configs/wb50n_defconfig
index e764c30..1b444f0 100644
--- a/configs/wb50n_defconfig
+++ b/configs/wb50n_defconfig
@@ -43,5 +43,6 @@
 CONFIG_PHYLIB=y
 CONFIG_PHY_MICREL=y
 CONFIG_PHY_MICREL_KSZ90X1=y
+CONFIG_ATMEL_USART=y
 CONFIG_LZMA=y
 CONFIG_OF_LIBFDT=y