Convert CONFIG_SYS_SPL_ARGS_ADDR to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_SPL_ARGS_ADDR

In doing so, we also consistently use this variable for SPL_OS_BOOT and
not CONFIG_SYS_FDT_BASE in some cases.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/stm32746g-eval_spl_defconfig b/configs/stm32746g-eval_spl_defconfig
index 989bc7a..55e44f8 100644
--- a/configs/stm32746g-eval_spl_defconfig
+++ b/configs/stm32746g-eval_spl_defconfig
@@ -34,6 +34,7 @@
 CONFIG_SPL_SYS_MALLOC_SIMPLE=y
 CONFIG_SPL_MTD_SUPPORT=y
 CONFIG_SPL_XIP_SUPPORT=y
+CONFIG_SYS_SPL_ARGS_ADDR=0x81c0000
 CONFIG_SPL_DM_RESET=y
 CONFIG_SYS_PROMPT="U-Boot > "
 CONFIG_SYS_PBSIZE=1050