sh: Do not try merging two 128MB PMB mappings

There is a logic error in pmb_merge() that means we will incorrectly try
to merge two 128MB PMB mappings into one mapping. However, 256MB isn't a
valid PMB map size and pmb_merge() will actually drop the second 128MB
mapping.

This patch allows my SDK7786 board to boot when configured with
CONFIG_MEMORY_SIZE=0x10000000.
Signed-off-by: Matt Fleming <matt@console-pimps.org>

arch/sh/mm/pmb.c

@@ -681,7 +681,7 @@ static void __init pmb_merge(struct pmb_entry *head)
 	/*
 	 * The merged page size must be valid.
 	 */
-	if (!pmb_size_valid(newsize))
+	if (!depth || !pmb_size_valid(newsize))
 		return;
 
 	head->flags &= ~PMB_SZ_MASK;