of/platform: Allow overlays to create platform devices from the root node

We'd like to apply overlays to the root node in KUnit so we can test
platform devices created as children of the root node.

On some architectures (powerpc), the root node isn't marked with
OF_POPULATED_BUS. If an overlay tries to modify the root node on these
platforms it will fail, while on other platforms, such as ARM, it will
succeed. This is because the root node is marked with OF_POPULATED_BUS
by of_platform_default_populate_init() calling
of_platform_default_populate() with NULL as the first argument.

Loosen the requirement here so that platform devices can be created for
nodes created as children of the root node via DT overlays even if the
platform bus wasn't populated for the root node.

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Cc: Saravana Kannan <saravanak@google.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
[sboyd@kernel.org: Folded in condition fix]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lore.kernel.org/r/20240718210513.3801024-2-sboyd@kernel.org