MTD: small physmap_of partition parsing fixes

Use of_get_next_child for proper ref counting as suggested by Stephen Rothwell
and remove add_mtd_partitions from parse_partitions to avoid duplicate
mtd device registration for RedBoot partitions.

Signed-off-by: Valentine Barshak <vbarshak@ru.mvista.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Heckled-for-on-IRC-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>