[POWERPC] Fix dt_mem_next_cell() to read the full address

dt_mem_next_cell() currently does of_read_ulong().  This does not
allow for the case where #size-cells and/or #address-cells = 2 on a
32-bit system, as it will end up reading 32 bits instead of the
expected 64.  Change it to use of_read_number instead and always
return a u64.

Signed-off-by: Becky Bruce <becky.bruce at freescale.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>