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/vyasa-rk3288_defconfig b/configs/vyasa-rk3288_defconfig
index 27c704f..9ae7381 100644
--- a/configs/vyasa-rk3288_defconfig
+++ b/configs/vyasa-rk3288_defconfig
@@ -29,6 +29,7 @@
 CONFIG_SPL_STACK_R=y
 CONFIG_SPL_STACK_R_MALLOC_SIMPLE_LEN=0x2000
 CONFIG_SPL_OS_BOOT=y
+CONFIG_SYS_SPL_ARGS_ADDR=0xffe5000
 CONFIG_SPL_FALCON_BOOT_MMCSD=y
 CONFIG_SYS_MMCSD_RAW_MODE_KERNEL_SECTOR=0x8800
 CONFIG_CMD_SPL=y