of: Properly set the OF_POPULATED_BUS flag on root node

of_platform_populate() takes a subset of the device tree and turns it
into a set of platform_devices. At the same time it sets the
OF_POPULATED_BUS flag in each bus nodes so that of_platform_depopulate()
can undo the operation at a later time. However, it doesn't set the flag
on the root of the population tree which means that dynamic modifications
of the device tree at runtime will not create/destroy devices correctly.

Fix of_platform_populate() to set the OF_POPULATED_BUS flag on the node
it is called with.

Signed-off-by: Grant Likely <grant.likely@linaro.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Cc: Pawel Moll <pawel.moll@arm.com>