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/imx28_xea_defconfig b/configs/imx28_xea_defconfig
index cab908e..ba7eeb7 100644
--- a/configs/imx28_xea_defconfig
+++ b/configs/imx28_xea_defconfig
@@ -20,6 +20,7 @@
 CONFIG_SPL_SPI_FLASH_SUPPORT=y
 CONFIG_SPL_SPI_SUPPORT=y
 CONFIG_SPL_PAYLOAD="u-boot.img"
+CONFIG_SYS_LOAD_ADDR=0x42000000
 CONFIG_FIT=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_USE_BOOTARGS=y