serial: uartps: Do not use static struct uart_driver out of probe()

cdns_uart_suspend()/resume() and remove() are using static reference
to struct uart_driver. Assign this reference to private data structure
as preparation step for dynamic struct uart_driver allocation.

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