powerpc/nvram: Search for nvram using compatible

As well as searching for nodes with type = "nvram", search for nodes
that have compatible = "nvram". This can't be converted into a single
call to of_find_compatible_node() with a non-NULL type, because that
searches for a node that has _both_ type & compatible = "nvram".

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>