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/syzygy_hub_defconfig b/configs/syzygy_hub_defconfig
index 0baf6dd..d632cb8 100644
--- a/configs/syzygy_hub_defconfig
+++ b/configs/syzygy_hub_defconfig
@@ -36,6 +36,7 @@
 CONFIG_SYS_SPL_MALLOC_SIZE=0x2000000
 CONFIG_SPL_FS_LOAD_ARGS_NAME="system.dtb"
 CONFIG_SPL_OS_BOOT=y
+CONFIG_SYS_SPL_ARGS_ADDR=0x10000000
 CONFIG_SPL_FALCON_BOOT_MMCSD=y
 CONFIG_SYS_MMCSD_RAW_MODE_KERNEL_SECTOR=0x0
 CONFIG_SYS_MAXARGS=32