tty: serial: uartlite: Add structure for private data

Add struct uartlite_data, to store the private data of the Uartlite
driver.

Signed-off-by: Tanvi Desai <tanvi.desai@xilinx.com>
Signed-off-by: Shubhrajyoti Datta <shubhrajyoti.datta@xilinx.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>