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/poplar_defconfig b/configs/poplar_defconfig
index 82833ff..8c7a565 100644
--- a/configs/poplar_defconfig
+++ b/configs/poplar_defconfig
@@ -8,6 +8,7 @@
 CONFIG_DEFAULT_DEVICE_TREE="hi3798cv200-poplar"
 CONFIG_IDENT_STRING="poplar"
 CONFIG_DISTRO_DEFAULTS=y
+CONFIG_SYS_LOAD_ADDR=0x800000
 # CONFIG_DISPLAY_CPUINFO is not set
 CONFIG_SYS_PROMPT="poplar# "
 CONFIG_CMD_MMC=y