serdev: Set fwnode for serdev devices

This allow fw_devlink to do dependency tracking for serdev devices.

Reported-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/lkml/03b70a8a-0591-f28b-a567-9d2f736f17e5@gmail.com/
Cc: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Saravana Kannan <saravanak@google.com>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Tested-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20230302023509.319903-1-saravanak@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>