Convert CONFIG_SYS_LOAD_ADDR to Kconfig

Now that we have consistent usage, migrate this symbol to Kconfig.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Rick Chen <rick@andestech.com>
diff --git a/configs/evb-ast2600_defconfig b/configs/evb-ast2600_defconfig
index 19b9210..4b6e596 100644
--- a/configs/evb-ast2600_defconfig
+++ b/configs/evb-ast2600_defconfig
@@ -15,6 +15,7 @@
 CONFIG_SPL=y
 # CONFIG_ARMV7_NONSEC is not set
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
+CONFIG_SYS_LOAD_ADDR=0x83000000
 CONFIG_FIT=y
 # CONFIG_LEGACY_IMAGE_FORMAT is not set
 CONFIG_USE_BOOTARGS=y