serial: uartps: Add the device_init_wakeup

Initialise the device wakeup.

The device_init_wakeup is needed for the wakeup to work by default.
Uart can be configured as the primary wakeup source so it is good to
enable wakeup by default.

The same functionality is enabled also by 8250_omap, atmel_serial,
omap-serial and stm32-usart.

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