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/rpi_3_b_plus_defconfig b/configs/rpi_3_b_plus_defconfig
index e34302d..e92d992 100644
--- a/configs/rpi_3_b_plus_defconfig
+++ b/configs/rpi_3_b_plus_defconfig
@@ -8,6 +8,7 @@
 CONFIG_ENV_SIZE=0x4000
 CONFIG_DEFAULT_DEVICE_TREE="bcm2837-rpi-3-b-plus"
 CONFIG_DISTRO_DEFAULTS=y
+CONFIG_SYS_LOAD_ADDR=0x1000000
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_USE_PREBOOT=y
 # CONFIG_DISPLAY_CPUINFO is not set