Convert CONFIG_SYS_FDT_LOAD_ADDR to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_FDT_LOAD_ADDR

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/Kconfig b/arch/sandbox/Kconfig
index 7606469..7cdbaef 100644
--- a/arch/sandbox/Kconfig
+++ b/arch/sandbox/Kconfig
@@ -68,4 +68,14 @@
 	default 32 if HOST_32BIT
 	default 64 if HOST_64BIT
 
+config SYS_FDT_LOAD_ADDR
+	hex "Address at which to load devicetree"
+	default 0x100
+	help
+	  With sandbox the devicetree is loaded into the emulated RAM. This sets
+	  the address that is used. There must be enough space at this address
+	  to load the full devicetree without it overwriting anything else.
+
+	  See `doc/arch/sandbox.rst` for more information.
+
 endmenu