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/at91sam9m10g45ek_nandflash_defconfig b/configs/at91sam9m10g45ek_nandflash_defconfig
index c242c5c..99540f8 100644
--- a/configs/at91sam9m10g45ek_nandflash_defconfig
+++ b/configs/at91sam9m10g45ek_nandflash_defconfig
@@ -5,6 +5,7 @@
 CONFIG_SYS_MALLOC_LEN=0x80000
 CONFIG_SYS_MALLOC_F_LEN=0x2000
 CONFIG_TARGET_AT91SAM9M10G45EK=y
+CONFIG_ATMEL_LEGACY=y
 CONFIG_NR_DRAM_BANKS=1
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="at91sam9m10g45ek"