arm: Add missing gFDT storage for FDT bus

Change-Id: I7c7eaa72515ecb5c2e50c089ceb7428b4bafbf22
Reviewed-on: https://review.haiku-os.org/c/haiku/+/4430
Reviewed-by: Alex von Gluck IV <kallisti5@unixzen.com>

src/system/kernel/arch/arm/arch_platform.cpp

@@ -11,11 +11,13 @@
 #include <boot/kernel_args.h>
 
 
+void* gFDT = NULL;
+
+
 status_t
 arch_platform_init(struct kernel_args *kernelArgs)
 {
-	// NOTE: dprintf() is off-limits here, too early...
-
+	gFDT = kernelArgs->arch_args.fdt;
 	return B_OK;
 }