of: module: Export of_device_uevent()

The content of of_device_uevent() is currently hardcoded in a driver
that can be compiled as a module. Nothing prevents of_device_uevent() to
be exported to modules, most of the other helpers in of/device.c
actually are. The reason why this helper was not exported is because it
has been so far only useful in drivers/base, which is built-in anyway.

With the idea of getting rid of the hardcoded implementation of
of_device_uevent() in other places in the kernel, let's export it to GPL
modules (very much like its cousins in the same file).

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230622213214.3586530-2-miquel.raynal@bootlin.com