tty: serial: uartlite: Add runtime pm support

tty: serial: uartlite: Add runtime pm support

In the commit 4f86e1506a0c ("Revert serial-uartlite: Add runtime
support") the runtime pm support was reverted to aid reverting of
the other patches.

This patch adds the runtime PM support back.
The runtime pm calls are used to gate and enable the clocks.

Signed-off-by: Shubhrajyoti Datta <shubhrajyoti.datta@xilinx.com>
Link: https://lore.kernel.org/r/20210713064835.27978-3-shubhrajyoti.datta@xilinx.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>