serial: uartps: Move register to probe based on run time detection

Register uart driver in probe to be able to register one device with
unique major/minor separately. Also calculate number of instances of
this driver to be able to call uart_unregister_driver() when there is no
instance.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>