serial: struct serial_device: add uninit() entry for drivers

Subsequent patch extends mpc512x serial driver to support
multiple PSC ports. The driver will provide an uninit()
function to stop the serial controller and to disable the
controller's clock. Adding uninit() entry to struct serial_device
allows disabling the serial controller after usage of
a stdio serial device.

This patch adds uninit() entry to the struct serial_device
and fixes initialization of this structure in the code
accordingly.

Signed-off-by: Anatolij Gustschin <agust@denx.de>