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/silk_defconfig b/configs/silk_defconfig
index 020e6b0..68005c8 100644
--- a/configs/silk_defconfig
+++ b/configs/silk_defconfig
@@ -23,6 +23,7 @@
 CONFIG_SPL=y
 CONFIG_SPL_SPI_FLASH_SUPPORT=y
 CONFIG_SPL_SPI_SUPPORT=y
+CONFIG_SYS_LOAD_ADDR=0x50000000
 CONFIG_FIT=y
 CONFIG_BOOTDELAY=3
 CONFIG_SPL_BOARD_INIT=y