Convert CONFIG_ATMEL_LEGACY to Kconfig

This converts the following to Kconfig:
   CONFIG_ATMEL_LEGACY

Cc: Eugen Hristev <eugen.hristev@microchip.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/gardena-smart-gateway-at91sam_defconfig b/configs/gardena-smart-gateway-at91sam_defconfig
index 97ed6709..450ff86 100644
--- a/configs/gardena-smart-gateway-at91sam_defconfig
+++ b/configs/gardena-smart-gateway-at91sam_defconfig
@@ -7,6 +7,7 @@
 CONFIG_SYS_MALLOC_LEN=0x1000000
 CONFIG_SYS_MALLOC_F_LEN=0x2000
 CONFIG_TARGET_GARDENA_SMART_GATEWAY_AT91SAM=y
+CONFIG_ATMEL_LEGACY=y
 CONFIG_SPL_GPIO=y
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
 CONFIG_SPL_LIBGENERIC_SUPPORT=y