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/bg0900_defconfig b/configs/bg0900_defconfig
index d5aeb97..ec8ffdb 100644
--- a/configs/bg0900_defconfig
+++ b/configs/bg0900_defconfig
@@ -10,6 +10,7 @@
 CONFIG_TARGET_BG0900=y
 CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SPL=y
+CONFIG_SYS_LOAD_ADDR=0x42000000
 CONFIG_BOOTDELAY=3
 CONFIG_USE_BOOTARGS=y
 CONFIG_BOOTARGS="console=ttyAMA0,115200"