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/mx23evk_defconfig b/configs/mx23evk_defconfig
index ab45a9c..f879f02 100644
--- a/configs/mx23evk_defconfig
+++ b/configs/mx23evk_defconfig
@@ -13,6 +13,7 @@
 CONFIG_TARGET_MX23EVK=y
 CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SPL=y
+CONFIG_SYS_LOAD_ADDR=0x42000000
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_ARCH_MISC_INIT=y