can: xilinx: Convert to runtime_pm

Instead of enabling/disabling clocks at several locations in the driver,
Use the runtime_pm framework. This consolidates the actions for runtime PM
In the appropriate callbacks and makes the driver more readable and mantainable.

Signed-off-by: Kedareswara rao Appana <appanad@xilinx.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>