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/mx28evk_auart_console_defconfig b/configs/mx28evk_auart_console_defconfig
index 1f61dda..2fb598f 100644
--- a/configs/mx28evk_auart_console_defconfig
+++ b/configs/mx28evk_auart_console_defconfig
@@ -11,6 +11,7 @@
 CONFIG_TARGET_MX28EVK=y
 CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SPL=y
+CONFIG_SYS_LOAD_ADDR=0x42000000
 CONFIG_SYS_EXTRA_OPTIONS="MXS_AUART,MXS_AUART_BASE=MXS_UARTAPP3_BASE"
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y
 # CONFIG_DISPLAY_BOARDINFO is not set