of/flattree: use callback to setup initrd from /chosen

At present, the fdt code sets the kernel-wide initrd_start and
initrd_end variables when parsing /chosen. On ARM, we only set these
once the bootmem has been reserved.

This change adds an arch hook to setup the initrd from the device
tree:

 void early_init_dt_setup_initrd_arch(unsigned long start,
				      unsigned long end);

The arch-specific code can then setup the initrd however it likes.

Compiled on powerpc, with CONFIG_BLK_DEV_INITRD=y and =n.

Signed-off-by: Jeremy Kerr <jeremy.kerr@canonical.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

arch/microblaze/kernel/prom.c

@@ -118,6 +118,16 @@ void __init early_init_devtree(void *params)
 	pr_debug(" <- early_init_devtree()\n");
 }
 
+#ifdef CONFIG_BLK_DEV_INITRD
+void __init early_init_dt_setup_initrd_arch(unsigned long start,
+		unsigned long end)
+{
+	initrd_start = (unsigned long)__va(start);
+	initrd_end = (unsigned long)__va(end);
+	initrd_below_start_ok = 1;
+}
+#endif
+
 /*******
  *
  * New implementation of the OF "find" APIs, return a refcounted