serdev: add of_ helper to get serdev controller

add of_find_serdev_controller_by_node to obtain a
serdev_controller from the device_node, which
can help if the serdev_device is not described
over device tree and instantiation of the device
happens from a different driver, for the same purpose
an option to not delete an empty serdev controller
is added.

Signed-off-by: Vaishnav M A <vaishnav@beagleboard.org>