[POWERPC] Don't complain if size-cells == 0 in prom_parse()

An mdio bus scan was added with ucc_geth phylib
migration patches, now machines complain on boot, saying:

prom_parse: Bad cell count for /qe@e0100000/mdio@2120/ethernet-phy@00
prom_parse: Bad cell count for /qe@e0100000/mdio@2120/ethernet-phy@01

since size-cells can indeed be 0, this patch fixes the check.

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>