clocksource: timer-ti-dm: Hook device platform data if not already assigned

In the case of device tree boot the device platform data is usually
NULL so hook the platform data obtained from the match.
As part of un-constify the platform_data pointer.

Signed-off-by: Ladislav Michl <ladis@linux-mips.org>
Signed-off-by: Keerthy <j-keerthy@ti.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Tony Lindgren <tony@atomide.com>