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/imxrt1020-evk_defconfig b/configs/imxrt1020-evk_defconfig
index e53c5ca..7eea093 100644
--- a/configs/imxrt1020-evk_defconfig
+++ b/configs/imxrt1020-evk_defconfig
@@ -16,6 +16,7 @@
 CONFIG_SPL_SIZE_LIMIT=0x20000
 CONFIG_SPL=y
 CONFIG_DISTRO_DEFAULTS=y
+CONFIG_SYS_LOAD_ADDR=0x20209000
 CONFIG_SD_BOOT=y
 # CONFIG_USE_BOOTCOMMAND is not set
 # CONFIG_DISPLAY_CPUINFO is not set