coresight: use const for device_node structures

Almost low level functions from open firmware have used const to
qualify device_node structures, so add const for device_node
parameters in of_coresight related functions.

Signed-off-by: Leo Yan <leo.yan@linaro.org>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>