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/picosam9g45_defconfig b/configs/picosam9g45_defconfig
index 2921f2d..03783a2 100644
--- a/configs/picosam9g45_defconfig
+++ b/configs/picosam9g45_defconfig
@@ -40,6 +40,7 @@
 CONFIG_DM=y
 CONFIG_SPL_DM=y
 CONFIG_AT91_GPIO=y
+CONFIG_ATMEL_USART=y
 CONFIG_USB=y
 CONFIG_USB_EHCI_HCD=y
 CONFIG_USB_STORAGE=y