powerpc/lmb-size: Use addr #size-cells value when fetching lmb-size

Make it consistent with other usages.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201007114836.282468-5-aneesh.kumar@linux.ibm.com