driver core: Add fwnode_init()

There are multiple locations in the kernel where a struct fwnode_handle
is initialized. Add fwnode_init() so that we have one way of
initializing a fwnode_handle.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Saravana Kannan <saravanak@google.com>
Link: https://lore.kernel.org/r/20201121020232.908850-8-saravanak@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>