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/sipeed_maix_bitm_defconfig b/configs/sipeed_maix_bitm_defconfig
index 33c67c0..eef43a2 100644
--- a/configs/sipeed_maix_bitm_defconfig
+++ b/configs/sipeed_maix_bitm_defconfig
@@ -4,6 +4,7 @@
 CONFIG_ENV_SECT_SIZE=0x1000
 CONFIG_TARGET_SIPEED_MAIX=y
 CONFIG_ARCH_RV64I=y
+CONFIG_SYS_LOAD_ADDR=0x80000000
 CONFIG_STACK_SIZE=0x100000
 CONFIG_USE_BOOTCOMMAND=y
 CONFIG_BOOTCOMMAND="run k210_bootcmd"