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/stm32h750-art-pi_defconfig b/configs/stm32h750-art-pi_defconfig
index 1af66c5..f8c3361 100644
--- a/configs/stm32h750-art-pi_defconfig
+++ b/configs/stm32h750-art-pi_defconfig
@@ -8,6 +8,7 @@
 CONFIG_STM32H7=y
 CONFIG_TARGET_STM32H750_ART_PI=y
 CONFIG_DISTRO_DEFAULTS=y
+CONFIG_SYS_LOAD_ADDR=0xc1800000
 CONFIG_FIT=y
 CONFIG_BOOTDELAY=3
 CONFIG_AUTOBOOT_KEYED=y