drivers: timer: ti_dmtimer: Do not require systick_timer node label

This driver currently only supports one instance of this timer and uses
it as the system clock. The instance is selected by being the first one
listed in DT in all places except sys_clock_driver_init() where it uses
the node label "systick_timer".

This driver should be fixed to correctly support multiple instances
of this timer, and the one used for the system timer should be selected
based on a flag or alias, not based on label.

For now simply use the 0th instance like everywhere else which removes
the need to have this node labeled a special way and makes no functional
changes to current users.

Signed-off-by: Andrew Davis <afd@ti.com>