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/xilinx_zynqmp_virt_defconfig b/configs/xilinx_zynqmp_virt_defconfig
index 0ac75f6..7f0ed8b 100644
--- a/configs/xilinx_zynqmp_virt_defconfig
+++ b/configs/xilinx_zynqmp_virt_defconfig
@@ -46,6 +46,7 @@
 CONFIG_SPL_FS_LOAD_ARGS_NAME="u-boot.bin"
 CONFIG_SPL_FPGA=y
 CONFIG_SPL_OS_BOOT=y
+CONFIG_SYS_SPL_ARGS_ADDR=0x8000000
 CONFIG_SPL_RAM_SUPPORT=y
 CONFIG_SPL_RAM_DEVICE=y
 CONFIG_SPL_SPI_LOAD=y