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/am3517_evm_defconfig b/configs/am3517_evm_defconfig
index 03fc4fb..05ee357 100644
--- a/configs/am3517_evm_defconfig
+++ b/configs/am3517_evm_defconfig
@@ -31,6 +31,7 @@
 CONFIG_SPL_NAND_SIMPLE=y
 CONFIG_SPL_NAND_BASE=y
 CONFIG_SPL_OS_BOOT=y
+CONFIG_SYS_SPL_ARGS_ADDR=0x84000000
 CONFIG_SPL_FALCON_BOOT_MMCSD=y
 CONFIG_SYS_MMCSD_RAW_MODE_KERNEL_SECTOR=0x1700
 # CONFIG_SPL_POWER is not set