tty: serial: uartlite: Enable clocks at probe

At probe the uartlite is getting configured.
Enable the clocks before assiging uart and
disable after probe is done.

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>