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/mccmon6_nor_defconfig b/configs/mccmon6_nor_defconfig
index 1214163..80b59cb 100644
--- a/configs/mccmon6_nor_defconfig
+++ b/configs/mccmon6_nor_defconfig
@@ -29,6 +29,7 @@
 CONFIG_SPL_ENV_SUPPORT=y
 CONFIG_SPL_NOR_SUPPORT=y
 CONFIG_SPL_OS_BOOT=y
+CONFIG_SYS_SPL_ARGS_ADDR=0x18000000
 CONFIG_SYS_OS_BASE=0x8180000
 CONFIG_SYS_MAXARGS=32
 CONFIG_SYS_PBSIZE=532