platform: introduce OF style 'modalias' support for platform bus

Fix a problem that, the platform bus supports the OF style modalias
in .uevent() call, but not in its device 'modalias' sysfs attribute.

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>