devicetree: fix dt_get_memory_params

Index nr was overreaching with its indexing. It should only
index the caller's regs array. We need a different index for
the memory nodes' reg properties.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Tested-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

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The next patch needs this fix.